diff --git a/.circleci/config.yml b/.circleci/config.yml
index 41610d84319..5cc1df36556 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -614,9 +614,9 @@ jobs:
                 //packages/common/locales
 
             # Needed for the ES5 downlevel reflector test in `packages/core/test/reflection`.
-            mkdir -p dist/all/@angular/core/test/reflection/
+            mkdir -p dist/legacy-test-out/core/test/reflection/
             cp dist/bin/packages/core/test/reflection/es5_downleveled_inheritance_fixture.js \
-              dist/all/@angular/core/test/reflection/es5_downleveled_inheritance_fixture.js
+              dist/legacy-test-out/core/test/reflection/es5_downleveled_inheritance_fixture.js
             # Locale files are needed for i18n tests running within Saucelabs. These are added
             # directly as sources so that the TypeScript compilation of `/packages/tsconfig.json`
             # can succeed. Note that the base locale and currencies files are checked-in, so
@@ -624,11 +624,10 @@ jobs:
             mkdir -p packages/common/locales/extra
             cp dist/bin/packages/common/locales/*.ts packages/common/locales
             cp dist/bin/packages/common/locales/extra/*.ts packages/common/locales/extra
-      # add module umd tsc compile option so the test can work
-      # properly in the legacy browsers
-      - run: yarn tsc -p packages/tsconfig-legacy-saucelabs.json --module UMD
-      - run: yarn tsc -p modules --module UMD
+      # Build ZoneJS so that it can be loaded globally in the Karma tests.
       - run: yarn bazel build //packages/zone.js:npm_package
+      # Build the bundle for all tests to run within Saucelabs
+      - run: node tools/legacy-saucelabs/build-saucelabs-test-bundle.mjs
       - run:
           # Waiting on ready ensures that we don't run tests too early without Saucelabs not being ready.
           name: Waiting for Saucelabs tunnel to connect
@@ -636,8 +635,7 @@ jobs:
       - run:
           name: Running tests on Saucelabs.
           command: |
-            browsers=$(node -e 'console.log(require("./browser-providers.conf").sauceAliases.CI_REQUIRED.join(","))')
-            yarn karma start ./karma-js.conf.js --single-run --browsers=${browsers}
+            KARMA_WEB_TEST_MODE=SL_REQUIRED yarn karma start ./karma-js.conf.js --single-run
       - run:
           name: Stop Saucelabs tunnel service
           command: ./tools/saucelabs/sauce-service.sh stop
diff --git a/.pullapprove.yml b/.pullapprove.yml
index 648739d8cb0..55498a1847f 100644
--- a/.pullapprove.yml
+++ b/.pullapprove.yml
@@ -1196,6 +1196,7 @@ groups:
           'tools/circular_dependency_test/**',
           'tools/contributing-stats/**',
           'tools/gulp-tasks/**',
+          'tools/legacy-saucelabs/**',
           'tools/npm/**',
           'tools/npm_integration_test/**',
           'tools/rxjs/**',
diff --git a/karma-js.conf.js b/karma-js.conf.js
index 575e202b31a..98588131ce5 100644
--- a/karma-js.conf.js
+++ b/karma-js.conf.js
@@ -23,10 +23,6 @@ module.exports = function(config) {
     },
 
     files: [
-      // Sources and specs.
-      // Loaded through the System loader, in `test-main.js`.
-      {pattern: 'dist/all/@angular/**/*.js', included: false, watched: true},
-
       // Serve AngularJS for `ngUpgrade` testing.
       {pattern: 'node_modules/angular-1.5/angular?(.min).js', included: false, watched: false},
       {pattern: 'node_modules/angular-mocks-1.5/angular-mocks.js', included: false, watched: false},
@@ -46,10 +42,9 @@ module.exports = function(config) {
       'dist/bin/packages/zone.js/npm_package/bundles/zone-testing.umd.js',
       'dist/bin/packages/zone.js/npm_package/bundles/task-tracking.umd.js',
 
-      // Including systemjs because it defines `__eval`, which produces correct stack traces.
-      'test-events.js',
-      'third_party/shims_for_internal_tests.js',
-      'node_modules/systemjs/dist/system.src.js',
+      // Static test assets.
+      {pattern: 'packages/platform-browser/test/static_assets/**/*', included: false},
+      {pattern: 'packages/platform-browser/test/browser/static_assets/**/*', included: false},
 
       // Serve polyfills necessary for testing the `elements` package.
       {
@@ -58,42 +53,9 @@ module.exports = function(config) {
         watched: false
       },
 
-      {pattern: 'node_modules/rxjs/**', included: false, watched: false, served: true},
       'node_modules/reflect-metadata/Reflect.js',
-      'tools/build/file2modulename.js',
-      'test-main.js',
-      {pattern: 'dist/all/@angular/empty.*', included: false, watched: false},
-      {pattern: 'packages/platform-browser/test/static_assets/**', included: false, watched: false},
-      {
-        pattern: 'packages/platform-browser/test/browser/static_assets/**',
-        included: false,
-        watched: false,
-      },
-    ],
 
-    exclude: [
-      'dist/all/@angular/_testing_init/**',
-      'dist/all/@angular/**/e2e_test/**',
-      'dist/all/@angular/**/*node_only_spec.js',
-      'dist/all/@angular/benchpress/**',
-      'dist/all/@angular/compiler-cli/**',
-      'dist/all/@angular/compiler-cli/src/ngtsc/**',
-      'dist/all/@angular/compiler-cli/test/compliance/**',
-      'dist/all/@angular/compiler-cli/test/ngtsc/**',
-      'dist/all/@angular/compiler/test/aot/**',
-      'dist/all/@angular/compiler/test/render3/**',
-      'dist/all/@angular/core/test/bundling/**',
-      'dist/all/@angular/core/test/render3/ivy/**',
-      'dist/all/@angular/core/test/render3/jit/**',
-      'dist/all/@angular/core/test/render3/perf/**',
-      'dist/all/@angular/elements/schematics/**',
-      'dist/all/@angular/examples/**/e2e_test/*',
-      'dist/all/@angular/language-service/**',
-      'dist/all/@angular/localize/**/test/**',
-      'dist/all/@angular/localize/schematics/**',
-      'dist/all/@angular/router/**/test/**',
-      'dist/all/@angular/platform-browser/testing/e2e_util.js',
-      'dist/examples/**/e2e_test/**',
+      'dist/legacy-test-bundle.spec.js',
     ],
 
     customLaunchers: browserProvidersConf.customLaunchers,
@@ -149,15 +111,6 @@ module.exports = function(config) {
     set: () => {},
   });
 
-  if (process.env.CIRCLECI) {
-    conf.frameworks.unshift('parallel');
-    conf.plugins.unshift(require('karma-parallel'));
-    conf.parallelOptions = {
-      executors: 2,
-      shardStrategy: 'round-robin',
-    };
-  }
-
   if (process.env['SAUCE_TUNNEL_IDENTIFIER']) {
     console.log(`SAUCE_TUNNEL_IDENTIFIER: ${process.env.SAUCE_TUNNEL_IDENTIFIER}`);
 
@@ -171,8 +124,7 @@ module.exports = function(config) {
     // the test logs from upstream and tries re-uploading them with the Karma enhanced details.
     // This slows-down tests/browser restarting and can decrease stability.
     // https://github.com/karma-runner/karma-sauce-launcher/blob/59b0c5c877448e064ad56449cd906743721c6b62/src/launcher/launcher.ts#L72-L79.
-    require('saucelabs').default.prototype.downloadJobAsset =
-        () => Promise.resolve('<FAKE-LOGS>');
+    require('saucelabs').default.prototype.downloadJobAsset = () => Promise.resolve('<FAKE-LOGS>');
   }
 
   // For SauceLabs jobs, we set up a domain which resolves to the machine which launched
diff --git a/modules/benchmarks/src/old/compiler/compiler_benchmark.ts b/modules/benchmarks/src/old/compiler/compiler_benchmark.ts
index 88b135b5609..4f57f7aa4b4 100644
--- a/modules/benchmarks/src/old/compiler/compiler_benchmark.ts
+++ b/modules/benchmarks/src/old/compiler/compiler_benchmark.ts
@@ -8,7 +8,7 @@
 
 import {CompilerConfig, DirectiveResolver} from '@angular/compiler';
 import {Component, ComponentResolver, Directive, ViewContainerRef,} from '@angular/core';
-import {ViewMetadata} from '@angular/core/src/metadata/view';
+import {ViewMetadata} from '@angular/core/view';
 import {PromiseWrapper} from '@angular/facade/src/async';
 import {print, Type} from '@angular/facade/src/lang';
 import {bootstrap} from '@angular/platform-browser';
diff --git a/package.json b/package.json
index 8c2a25dbda9..dc0dbc43c6d 100644
--- a/package.json
+++ b/package.json
@@ -176,9 +176,9 @@
     "gulp-conventional-changelog": "^2.0.35",
     "husky": "7.0.4",
     "inquirer": "^8.0.0",
-    "karma-parallel": "^0.3.1",
     "karma-sauce-launcher": "^4.3.6",
     "madge": "^5.0.0",
+    "multimatch": "^6.0.0",
     "sauce-connect": "https://saucelabs.com/downloads/sc-4.7.1-linux.tar.gz",
     "semver": "^7.3.5",
     "ts-node": "^10.0.0",
diff --git a/packages/core/test/acceptance/content_spec.ts b/packages/core/test/acceptance/content_spec.ts
index c30171fc2bc..2b264f4b405 100644
--- a/packages/core/test/acceptance/content_spec.ts
+++ b/packages/core/test/acceptance/content_spec.ts
@@ -7,8 +7,7 @@
  */
 
 import {CommonModule} from '@angular/common';
-import {ChangeDetectorRef, Component, Directive, TemplateRef, ViewChild, ViewContainerRef} from '@angular/core';
-import {Input} from '@angular/core/src/metadata';
+import {ChangeDetectorRef, Component, Directive, Input, TemplateRef, ViewChild, ViewContainerRef} from '@angular/core';
 import {TestBed} from '@angular/core/testing';
 import {By} from '@angular/platform-browser';
 import {expect} from '@angular/platform-browser/testing/src/matchers';
@@ -322,7 +321,7 @@ describe('projection', () => {
        @Component(
            {selector: 'comp', template: `<ng-template><ng-content></ng-content></ng-template>`})
        class Comp {
-         @ViewChild(TemplateRef, {static: true}) template !: TemplateRef<any>;
+         @ViewChild(TemplateRef, {static: true}) template!: TemplateRef<any>;
        }
 
        @Directive({selector: '[trigger]'})
diff --git a/packages/core/test/acceptance/directive_spec.ts b/packages/core/test/acceptance/directive_spec.ts
index 135e92274f1..b2dce9f0241 100644
--- a/packages/core/test/acceptance/directive_spec.ts
+++ b/packages/core/test/acceptance/directive_spec.ts
@@ -7,8 +7,7 @@
  */
 
 import {CommonModule} from '@angular/common';
-import {Component, Directive, ElementRef, EventEmitter, NgModule, Output, TemplateRef, ViewChild, ViewContainerRef} from '@angular/core';
-import {Input} from '@angular/core/src/metadata';
+import {Component, Directive, ElementRef, EventEmitter, Input, NgModule, Output, TemplateRef, ViewChild, ViewContainerRef} from '@angular/core';
 import {TestBed} from '@angular/core/testing';
 import {By} from '@angular/platform-browser';
 
diff --git a/packages/core/test/acceptance/discover_utils_spec.ts b/packages/core/test/acceptance/discover_utils_spec.ts
index f7c0450c312..6ce5b4b81a3 100644
--- a/packages/core/test/acceptance/discover_utils_spec.ts
+++ b/packages/core/test/acceptance/discover_utils_spec.ts
@@ -6,10 +6,8 @@
  * found in the LICENSE file at https://angular.io/license
  */
 import {CommonModule} from '@angular/common';
-import {Component, Directive, HostBinding, InjectionToken, ViewChild} from '@angular/core';
-import {ChangeDetectionStrategy} from '@angular/core/src/change_detection';
+import {ChangeDetectionStrategy, Component, Directive, HostBinding, InjectionToken, Input, Output, ViewChild, ViewEncapsulation} from '@angular/core';
 import {EventEmitter} from '@angular/core/src/event_emitter';
-import {Input, Output, ViewEncapsulation} from '@angular/core/src/metadata';
 import {isLView} from '@angular/core/src/render3/interfaces/type_checks';
 import {CONTEXT} from '@angular/core/src/render3/interfaces/view';
 import {ComponentFixture, TestBed} from '@angular/core/testing';
diff --git a/packages/core/test/acceptance/text_spec.ts b/packages/core/test/acceptance/text_spec.ts
index 952006c5a9a..c08efd47a5e 100644
--- a/packages/core/test/acceptance/text_spec.ts
+++ b/packages/core/test/acceptance/text_spec.ts
@@ -127,7 +127,8 @@ describe('text instructions', () => {
     fixture.detectChanges();
     const div = fixture.nativeElement.querySelector('div');
 
-    expect(div.innerHTML).toBe('function foo() { }');
+    expect(div.innerHTML).toBe(fixture.componentInstance.test.toString());
+    expect(div.innerHTML).toContain('foo');
   });
 
   it('should stringify an object using its toString method', () => {
diff --git a/packages/core/test/acceptance/view_container_ref_spec.ts b/packages/core/test/acceptance/view_container_ref_spec.ts
index 673dc252a6f..4a2a52e77a5 100644
--- a/packages/core/test/acceptance/view_container_ref_spec.ts
+++ b/packages/core/test/acceptance/view_container_ref_spec.ts
@@ -8,8 +8,7 @@
 
 import {CommonModule, DOCUMENT} from '@angular/common';
 import {computeMsgId} from '@angular/compiler';
-import {Compiler, Component, ComponentFactoryResolver, Directive, DoCheck, ElementRef, EmbeddedViewRef, ErrorHandler, InjectionToken, Injector, NgModule, NgModuleRef, NO_ERRORS_SCHEMA, OnDestroy, OnInit, Pipe, PipeTransform, QueryList, RendererFactory2, RendererType2, Sanitizer, TemplateRef, ViewChild, ViewChildren, ViewContainerRef, ɵsetDocument} from '@angular/core';
-import {Input} from '@angular/core/src/metadata';
+import {Compiler, Component, ComponentFactoryResolver, Directive, DoCheck, ElementRef, EmbeddedViewRef, ErrorHandler, InjectionToken, Injector, Input, NgModule, NgModuleRef, NO_ERRORS_SCHEMA, OnDestroy, OnInit, Pipe, PipeTransform, QueryList, RendererFactory2, RendererType2, Sanitizer, TemplateRef, ViewChild, ViewChildren, ViewContainerRef, ɵsetDocument} from '@angular/core';
 import {ngDevModeResetPerfCounters} from '@angular/core/src/util/ng_dev_mode';
 import {ComponentFixture, TestBed, TestComponentRenderer} from '@angular/core/testing';
 import {clearTranslations, loadTranslations} from '@angular/localize';
diff --git a/packages/core/test/animation/animations_with_web_animations_integration_spec.ts b/packages/core/test/animation/animations_with_web_animations_integration_spec.ts
index a4f1f526654..c38ae02d3a6 100644
--- a/packages/core/test/animation/animations_with_web_animations_integration_spec.ts
+++ b/packages/core/test/animation/animations_with_web_animations_integration_spec.ts
@@ -249,7 +249,8 @@ describe('animation integration tests using web animations', function() {
                 overflow:hidden;
               }
               .list .inner {
-                line-height:50px;
+                box-sizing: border-box;
+                height: 50px;
               }
             `],
           template: `
diff --git a/packages/core/test/debug/debug_node_spec.ts b/packages/core/test/debug/debug_node_spec.ts
index f9f4427f4f3..41360e75a6c 100644
--- a/packages/core/test/debug/debug_node_spec.ts
+++ b/packages/core/test/debug/debug_node_spec.ts
@@ -8,8 +8,7 @@
 
 
 import {CommonModule, NgIfContext, ɵgetDOM as getDOM} from '@angular/common';
-import {Component, DebugElement, DebugNode, Directive, ElementRef, EmbeddedViewRef, EventEmitter, HostBinding, Injectable, Input, NO_ERRORS_SCHEMA, OnInit, Output, Renderer2, TemplateRef, ViewChild, ViewContainerRef} from '@angular/core';
-import {NgZone} from '@angular/core/src/zone';
+import {Component, DebugElement, DebugNode, Directive, ElementRef, EmbeddedViewRef, EventEmitter, HostBinding, Injectable, Input, NgZone, NO_ERRORS_SCHEMA, OnInit, Output, Renderer2, TemplateRef, ViewChild, ViewContainerRef} from '@angular/core';
 import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';
 import {By} from '@angular/platform-browser/src/dom/debug/by';
 import {createMouseEvent, hasClass} from '@angular/platform-browser/testing/src/browser_util';
diff --git a/packages/core/test/directive_lifecycle_integration_spec.ts b/packages/core/test/directive_lifecycle_integration_spec.ts
index 1bddcba6c7e..b9408fae4ef 100644
--- a/packages/core/test/directive_lifecycle_integration_spec.ts
+++ b/packages/core/test/directive_lifecycle_integration_spec.ts
@@ -6,8 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {AfterContentChecked, AfterContentInit, AfterViewChecked, AfterViewInit, DoCheck, OnChanges, OnInit} from '@angular/core';
-import {Component, Directive} from '@angular/core/src/metadata';
+import {AfterContentChecked, AfterContentInit, AfterViewChecked, AfterViewInit, Component, Directive, DoCheck, OnChanges, OnInit} from '@angular/core';
 import {inject, TestBed} from '@angular/core/testing';
 import {Log} from '@angular/core/testing/src/testing_internal';
 
diff --git a/packages/core/test/linker/integration_spec.ts b/packages/core/test/linker/integration_spec.ts
index 657bede656e..a348d7bf843 100644
--- a/packages/core/test/linker/integration_spec.ts
+++ b/packages/core/test/linker/integration_spec.ts
@@ -7,7 +7,7 @@
  */
 
 import {CommonModule, DOCUMENT, ɵgetDOM as getDOM} from '@angular/common';
-import {Compiler, ComponentFactory, ComponentRef, EventEmitter, Host, Inject, Injectable, InjectionToken, Injector, NgModule, NgModuleRef, NO_ERRORS_SCHEMA, OnDestroy, SkipSelf, ViewChild, ViewRef} from '@angular/core';
+import {Attribute, Compiler, Component, ComponentFactory, ComponentRef, ContentChildren, Directive, EventEmitter, Host, HostBinding, HostListener, Inject, Injectable, InjectionToken, Injector, Input, NgModule, NgModuleRef, NO_ERRORS_SCHEMA, OnDestroy, Output, Pipe, SkipSelf, ViewChild, ViewRef} from '@angular/core';
 import {ChangeDetectionStrategy, ChangeDetectorRef, PipeTransform} from '@angular/core/src/change_detection/change_detection';
 import {ComponentFactoryResolver} from '@angular/core/src/linker/component_factory_resolver';
 import {ElementRef} from '@angular/core/src/linker/element_ref';
@@ -15,7 +15,6 @@ import {QueryList} from '@angular/core/src/linker/query_list';
 import {TemplateRef} from '@angular/core/src/linker/template_ref';
 import {ViewContainerRef} from '@angular/core/src/linker/view_container_ref';
 import {EmbeddedViewRef} from '@angular/core/src/linker/view_ref';
-import {Attribute, Component, ContentChildren, Directive, HostBinding, HostListener, Input, Output, Pipe} from '@angular/core/src/metadata';
 import {fakeAsync, getTestBed, TestBed, tick, waitForAsync} from '@angular/core/testing';
 import {createMouseEvent, dispatchEvent, el, isCommentNode} from '@angular/platform-browser/testing/src/browser_util';
 import {expect} from '@angular/platform-browser/testing/src/matchers';
diff --git a/packages/core/test/render3/i18n_debug_spec.ts b/packages/core/test/render3/i18n_debug_spec.ts
index 5d1f2d7766d..5ddcf7151fd 100644
--- a/packages/core/test/render3/i18n_debug_spec.ts
+++ b/packages/core/test/render3/i18n_debug_spec.ts
@@ -29,6 +29,13 @@ describe('i18n debug', () => {
     });
 
     it('should print Attribute opCode', () => {
+      // The `sanitizeFn` is written as actual function, compared to it being an arrow function.
+      // This is done to make this test less reluctant to build process changes where e.g. an
+      // arrow function might be transformed to a function declaration in ES5.
+      const sanitizeFn = function(v: any) {
+        return v;
+      };
+
       expect(i18nUpdateOpCodesToString([
         0b01,    8,
         'pre ',  -4,
@@ -39,11 +46,12 @@ describe('i18n debug', () => {
         'pre ',  -4,
         ' in ',  -3,
         ' post', 1 << I18nUpdateOpCode.SHIFT_REF | I18nUpdateOpCode.Attr,
-        'title', (v: any) => v,
+        'title', sanitizeFn,
       ] as unknown as I18nUpdateOpCodes))
           .toEqual([
             'if (mask & 0b1) { (lView[1] as Element).setAttribute(\'title\', `pre ${lView[i-4]} in ${lView[i-3]} post`); }',
-            'if (mask & 0b10) { (lView[1] as Element).setAttribute(\'title\', (function (v) { return v; })(`pre ${lView[i-4]} in ${lView[i-3]} post`)); }'
+            `if (mask & 0b10) { (lView[1] as Element).setAttribute('title', (${
+                sanitizeFn.toString()})(\`pre $\{lView[i-4]} in $\{lView[i-3]} post\`)); }`
           ]);
     });
 
diff --git a/packages/core/test/render3/ivy/jit_spec.ts b/packages/core/test/render3/ivy/jit_spec.ts
index c26f12478af..d1a0bfd6f7d 100644
--- a/packages/core/test/render3/ivy/jit_spec.ts
+++ b/packages/core/test/render3/ivy/jit_spec.ts
@@ -7,14 +7,10 @@
  */
 import 'reflect-metadata';
 
-import {ElementRef, QueryList} from '@angular/core';
+import {Component, ContentChild, ContentChildren, Directive, ElementRef, HostBinding, HostListener, Input, NgModule, Pipe, QueryList, ViewChild, ViewChildren, ɵNgModuleDef as NgModuleDef} from '@angular/core';
 import {Injectable} from '@angular/core/src/di/injectable';
 import {setCurrentInjector, ɵɵinject} from '@angular/core/src/di/injector_compatibility';
 import {ɵɵdefineInjectable, ɵɵInjectorDef} from '@angular/core/src/di/interface/defs';
-import {ContentChild, ContentChildren, ViewChild, ViewChildren} from '@angular/core/src/metadata/di';
-import {Component, Directive, HostBinding, HostListener, Input, Pipe} from '@angular/core/src/metadata/directives';
-import {NgModule} from '@angular/core/src/metadata/ng_module';
-import {NgModuleDef} from '@angular/core/src/metadata/ng_module_def';
 import {FactoryFn} from '@angular/core/src/render3/definition_factory';
 import {ComponentDef, PipeDef} from '@angular/core/src/render3/interfaces/definition';
 
diff --git a/packages/router/test/router_preloader.spec.ts b/packages/router/test/router_preloader.spec.ts
index 94d5c620b6e..03dc0c2f6ab 100644
--- a/packages/router/test/router_preloader.spec.ts
+++ b/packages/router/test/router_preloader.spec.ts
@@ -7,7 +7,6 @@
  */
 
 import {Compiler, Component, Injector, NgModule, NgModuleFactory, NgModuleRef, Type} from '@angular/core';
-import {resolveComponentResources} from '@angular/core/src/metadata/resource_loading';
 import {fakeAsync, inject, TestBed, tick} from '@angular/core/testing';
 import {PreloadAllModules, PreloadingStrategy, RouterPreloader} from '@angular/router';
 import {BehaviorSubject, Observable, of, throwError} from 'rxjs';
diff --git a/packages/tsconfig-legacy-saucelabs.json b/packages/tsconfig-legacy-saucelabs.json
index ec661240ab6..7d73f05d065 100644
--- a/packages/tsconfig-legacy-saucelabs.json
+++ b/packages/tsconfig-legacy-saucelabs.json
@@ -1,13 +1,37 @@
 {
   "extends": "./tsconfig.json",
   "compilerOptions": {
-    "target": "ES5",
-    "module": "commonjs",
+    "outDir": "../dist/legacy-test-out",
+    "target": "ES2015",
+    "module": "ES2020",
+    "importHelpers": true,
     // The project uses Bazel for TypeScript compilation. Unlike with Bazel, we build all
     // sources as part of a single TypeScript compilation. This results in `@internal`
     // declarations not being omitted between the logical as defined per the Bazel targets.
     // This can cause issues where the `override` keyword is needed for the legacy TS
     // compilation, but not within Bazel where the overridden member has `@internal`.
-    "noImplicitOverride": false
-  }
+    "noImplicitOverride": false,
+    // We run the decorator downlevel transform when compiling the sources and tests.
+    // Given that is the case, we do not need additional `design` metadata being emitted.
+    // The default TS decorator metadata would also not work with ES2015 JIT.
+    // https://github.com/angular/angular/issues/30106.
+    "emitDecoratorMetadata": false
+  },
+  "exclude": [
+    // Exclusions from the parent `tsconfig` need to be merged manually here.
+    "bazel",
+    "common/locales",
+    "compiler-cli/integrationtest",
+    "compiler-cli/test/compliance",
+    "core/schematics",
+    "elements/schematics",
+    "examples/**",
+    "http/**",
+    "platform-server/integrationtest",
+    "router/test/aot_ngsummary_test",
+
+    // Additional exclusion since tests for the language-service never run within the
+    // Saucelabs job and the package is not compatible with ESM TS compilation anyway.
+    "language-service/**",
+  ]
 }
diff --git a/packages/upgrade/static/test/integration/downgrade_module_spec.ts b/packages/upgrade/static/test/integration/downgrade_module_spec.ts
index 1b2f4b9f4f0..e686bb2659a 100644
--- a/packages/upgrade/static/test/integration/downgrade_module_spec.ts
+++ b/packages/upgrade/static/test/integration/downgrade_module_spec.ts
@@ -573,7 +573,9 @@ withEachNg1Version(() => {
                angular.module_('ng1', [lazyModuleName])
                    .directive(
                        'ng2', downgradeComponent({component: Ng2AComponent, propagateDigest}))
-                   .run(($rootScope: angular.IRootScopeService) => $rootScope.value = 0);
+                   .run([
+                     '$rootScope', ($rootScope: angular.IRootScopeService) => $rootScope.value = 0
+                   ]);
 
            const element = html('<div><ng2 [value]="value" ng-if="loadNg2"></ng2></div>');
            const $injector = angular.bootstrap(element, [ng1Module.name]);
@@ -762,10 +764,13 @@ withEachNg1Version(() => {
            const ng1Module =
                angular.module_('ng1', [lazyModuleName])
                    .directive('ng2', downgradeComponent({component: Ng2Component, propagateDigest}))
-                   .run(($rootScope: angular.IRootScopeService) => {
-                     $rootScope.attrVal = 'bar';
-                     $rootScope.propVal = 'bar';
-                   });
+                   .run([
+                     '$rootScope',
+                     ($rootScope: angular.IRootScopeService) => {
+                       $rootScope.attrVal = 'bar';
+                       $rootScope.propVal = 'bar';
+                     }
+                   ]);
 
            const element = html('<ng2 attr-input="{{ attrVal }}" [prop-input]="propVal"></ng2>');
            const $injector = angular.bootstrap(element, [ng1Module.name]);
diff --git a/test-main.js b/test-main.js
deleted file mode 100644
index 6e8ca29aa7d..00000000000
--- a/test-main.js
+++ /dev/null
@@ -1,213 +0,0 @@
-/**
- * @license
- * Copyright Google LLC All Rights Reserved.
- *
- * Use of this source code is governed by an MIT-style license that can be
- * found in the LICENSE file at https://angular.io/license
- */
-
-// Tun on full stack traces in errors to help debugging
-Error.stackTraceLimit = Infinity;
-
-jasmine.DEFAULT_TIMEOUT_INTERVAL = 15000;
-
-// Cancel Karma's synchronous start,
-// we will call `__karma__.start()` later, once all the specs are loaded.
-__karma__.loaded = function() {};
-
-window.isNode = false;
-window.isBrowser = true;
-
-System.config({
-  baseURL: '/base',
-  defaultJSExtensions: true,
-  map: {
-    'benchpress/*': 'dist/js/dev/es5/benchpress/*.js',
-    '@angular': 'dist/all/@angular',
-    'domino': 'dist/all/@angular/empty.js',
-    'url': 'dist/all/@angular/empty.js',
-    'xhr2': 'dist/all/@angular/empty.js',
-    '@angular/platform-server/src/domino_adapter': 'dist/all/@angular/empty.js',
-    'angular-in-memory-web-api': 'dist/all/@angular/misc/angular-in-memory-web-api',
-    'rxjs': 'node_modules/rxjs',
-  },
-  packages: {
-    '@angular/core/src/render3': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/core/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/core': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/animations/browser/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/animations/browser': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/animations/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/animations': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/compiler/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/compiler': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/common/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/common/http/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/common/http': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/common': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/forms': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/misc/angular-in-memory-web-api': {main: 'index.js', defaultExtension: 'js'},
-    // remove after all tests imports are fixed
-    '@angular/facade': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/router/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/router': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/localize/src/utils': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/localize/src/localize': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/localize/init': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/localize': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/upgrade/static/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/upgrade/static': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/upgrade': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/platform-browser/animations/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/platform-browser/animations': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/platform-browser/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/platform-browser': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/platform-browser-dynamic/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/platform-browser-dynamic': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/platform-server/init': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/platform-server/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/platform-server': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/private/testing': {main: 'index.js', defaultExtension: 'js'},
-    '@angular/elements': {main: 'index.js', defaultExtension: 'js'},
-    'rxjs/ajax': {main: 'index.js', defaultExtension: 'js'},
-    'rxjs/operators': {main: 'index.js', defaultExtension: 'js'},
-    'rxjs/testing': {main: 'index.js', defaultExtension: 'js'},
-    'rxjs/websocket': {main: 'index.js', defaultExtension: 'js'},
-    'rxjs': {main: 'index.js', defaultExtension: 'js'},
-  }
-});
-
-
-// Load browser-specific CustomElement polyfills, set up the test injector, import all the specs,
-// execute their `main()` method and kick off Karma (Jasmine).
-Promise
-    .resolve()
-
-    // Load browser-specific polyfills for custom elements.
-    .then(function() {
-      return loadCustomElementsPolyfills();
-    })
-
-    // Load necessary testing packages.
-    .then(function() {
-      return Promise.all([
-        System.import('@angular/core/testing'),
-        System.import('@angular/platform-browser-dynamic/testing'),
-        System.import('@angular/platform-browser/animations')
-      ]);
-    })
-
-    // Set up the test injector.
-    .then(function(mods) {
-      var coreTesting = mods[0];
-      var pbdTesting = mods[1];
-      var pbAnimations = mods[2];
-
-      coreTesting.TestBed.initTestEnvironment(
-          [pbdTesting.BrowserDynamicTestingModule, pbAnimations.NoopAnimationsModule],
-          pbdTesting.platformBrowserDynamicTesting());
-    })
-
-    // Import all the specs and execute their `main()` method.
-    .then(function() {
-      return Promise.all(Object
-                             .keys(window.__karma__.files)  // All files served by Karma.
-                             .filter(onlySpecFiles)
-                             .map(window.file2moduleName)  // Normalize paths to module names.
-                             .map(function(path) {
-                               return System.import(path).then(function(module) {
-                                 if (module.hasOwnProperty('main')) {
-                                   throw new Error('main() in specs are no longer supported');
-                                 }
-                               });
-                             }));
-    })
-
-    // Kick off karma (Jasmine).
-    .then(
-        function() {
-          __karma__.start();
-        },
-        function(error) {
-          console.error(error);
-        });
-
-
-function loadCustomElementsPolyfills() {
-  // The custom elements polyfill will patch properties and methods on `(HTML)Element` and `Node`
-  // (among others), including `(HTML)Element#innerHTML` and `Node#removeChild()`:
-  // https://github.com/webcomponents/custom-elements/blob/4f7072c0dbda4beb505d16967acfffd33337b325/src/Patch/Element.js#L28-L73
-  // https://github.com/webcomponents/custom-elements/blob/4f7072c0dbda4beb505d16967acfffd33337b325/src/Patch/Node.js#L105-L120
-  // The patched `innerHTML` setter and `removeChild()` method will try to traverse the DOM (via
-  // `nextSibling` and `parentNode` respectively), which leads to infinite loops when testing
-  // `HtmlSanitizer` with cloberred elements on browsers that do not support the `<template>`
-  // element:
-  // https://github.com/angular/angular/blob/213baa37b0b71e72d00ad7b606ebfc2ade06b934/packages/platform-browser/src/security/html_sanitizer.ts#L29-L38
-  // To avoid that, we "unpatch" these properties/methods and apply the patch only for the relevant
-  // `@angular/elements` tests.
-  var patchConfig = {'innerHTML': ['Element', 'HTMLElement'], 'removeChild': ['Node']};
-  var patchTargets = {};
-  var originalDescriptors = {};
-  if (!window.customElements) {
-    Object.keys(patchConfig).forEach(function(prop) {
-      patchConfig[prop]
-          .map(function(name) {
-            return window[name].prototype;
-          })
-          .some(function(candidatePatchTarget) {
-            var candidateOriginalDescriptor =
-                Object.getOwnPropertyDescriptor(candidatePatchTarget, prop);
-
-            if (candidateOriginalDescriptor) {
-              patchTargets[prop] = candidatePatchTarget;
-              originalDescriptors[prop] = candidateOriginalDescriptor;
-              return true;
-            }
-          });
-    });
-  }
-
-  var polyfillPath = !window.customElements ?
-      // Load custom elements polyfill.
-      'node_modules/@webcomponents/custom-elements/custom-elements.min.js' :
-      // Allow ES5 functions as custom element constructors.
-      'node_modules/@webcomponents/custom-elements/src/native-shim.js';
-
-  return System.import(polyfillPath).then(function() {
-    // `packages/compiler/test/schema/schema_extractor.ts` relies on
-    // `HTMLElement.name`, but custom element polyfills will replace
-    // `HTMLElement` with an anonymous function.
-    Object.defineProperty(HTMLElement, 'name', {value: 'HTMLElement'});
-
-    // Create helper functions on `window` for patching/restoring
-    // properties/methods.
-    Object.keys(patchConfig).forEach(function(prop) {
-      var patchMethod = '$$patch_' + prop;
-      var restoreMethod = '$$restore_' + prop;
-
-      if (!patchTargets[prop]) {
-        // No patching detected. Create no-op functions.
-        window[patchMethod] = window[restoreMethod] = function() {};
-      } else {
-        var patchTarget = patchTargets[prop];
-        var originalDescriptor = originalDescriptors[prop];
-        var patchedDescriptor = Object.getOwnPropertyDescriptor(patchTarget, prop);
-
-        window[patchMethod] = function() {
-          Object.defineProperty(patchTarget, prop, patchedDescriptor);
-        };
-        window[restoreMethod] = function() {
-          Object.defineProperty(patchTarget, prop, originalDescriptor);
-        };
-
-        // Restore `prop`. The patch will be manually applied only during the
-        // `@angular/elements` tests that need it.
-        window[restoreMethod]();
-      }
-    });
-  });
-}
-
-function onlySpecFiles(path) {
-  return /_spec\.js$/.test(path);
-}
diff --git a/tools/legacy-saucelabs/build-saucelabs-test-bundle.mjs b/tools/legacy-saucelabs/build-saucelabs-test-bundle.mjs
new file mode 100644
index 00000000000..e26741c2353
--- /dev/null
+++ b/tools/legacy-saucelabs/build-saucelabs-test-bundle.mjs
@@ -0,0 +1,267 @@
+#!/usr/bin/env node
+
+import multimatch from 'multimatch';
+import esbuild from 'esbuild';
+import fs from 'fs';
+import glob from 'glob';
+import {dirname, join, isAbsolute, relative} from 'path';
+import url from 'url';
+import ts from 'typescript';
+
+const containingDir = dirname(url.fileURLToPath(import.meta.url));
+const projectDir = join(containingDir, '../../');
+
+const distDir = join(projectDir, 'dist/');
+const packagesDir = join(projectDir, 'packages/');
+
+const legacyTsconfigPath = join(packagesDir, 'tsconfig-legacy-saucelabs.json');
+const legacyOutputDir = join(distDir, 'legacy-test-out');
+
+const outFile = join(distDir, 'legacy-test-bundle.spec.js');
+const decoratorDownlevelOutFile = join(distDir, 'legacy-decorator-downlevel-bundle.mjs');
+
+/**
+ * This script builds the whole library in `angular/angular` together with its
+ * spec files into a single IIFE bundle.
+ *
+ * The bundle can then be used in the legacy Saucelabs or Browserstack tests. Bundling
+ * helps with avoiding unnecessary complexity with maintaining module resolution at
+ * runtime through e.g. SystemJS, and it increases test stability by serving significantly
+ * less files through the remote browser service tunnels.
+ */
+async function main() {
+  await transpileDecoratorDownlevelTransform();
+  await compileProjectWithTsc();
+
+  const specEntryPointFile = await createEntryPointSpecFile();
+  const esbuildResolvePlugin = await createResolveEsbuildPlugin();
+
+  const result = await esbuild.build({
+    bundle: true,
+    keepNames: true,
+    treeShaking: false,
+    sourceRoot: projectDir,
+    platform: 'browser',
+    target: 'es2015',
+    format: 'iife',
+    outfile: outFile,
+    sourcemap: true,
+    plugins: [esbuildResolvePlugin],
+    stdin: {contents: specEntryPointFile, resolveDir: projectDir},
+  });
+
+  if (result.errors.length) {
+    throw Error('Could not build legacy test bundle. See errors above.');
+  }
+}
+
+/**
+ * Finds spec files that should be built, bundled and tested as
+ * part of the legacy Saucelabs test job.
+ */
+async function findSpecFiles() {
+  const baseTestFiles = glob.sync('**/*_spec.js', {absolute: true, cwd: legacyOutputDir});
+
+  return multimatch(
+    baseTestFiles,
+    [
+      '**/*',
+      `!${legacyOutputDir}/_testing_init/**`,
+      `!${legacyOutputDir}/**/e2e_test/**`,
+      `!${legacyOutputDir}/**/*node_only_spec.js`,
+      `!${legacyOutputDir}/benchpress/**`,
+      `!${legacyOutputDir}/compiler-cli/**`,
+      `!${legacyOutputDir}/compiler-cli/src/ngtsc/**`,
+      `!${legacyOutputDir}/compiler-cli/test/compliance/**`,
+      `!${legacyOutputDir}/compiler-cli/test/ngtsc/**`,
+      `!${legacyOutputDir}/compiler/test/aot/**`,
+      `!${legacyOutputDir}/compiler/test/render3/**`,
+      `!${legacyOutputDir}/core/test/bundling/**`,
+      `!${legacyOutputDir}/core/schematics/test/**`,
+      `!${legacyOutputDir}/core/test/render3/ivy/**`,
+      `!${legacyOutputDir}/core/test/render3/jit/**`,
+      `!${legacyOutputDir}/core/test/render3/perf/**`,
+      `!${legacyOutputDir}/elements/schematics/**`,
+      `!${legacyOutputDir}/examples/**/e2e_test/*`,
+      `!${legacyOutputDir}/language-service/**`,
+      `!${legacyOutputDir}/localize/**/test/**`,
+      `!${legacyOutputDir}/localize/schematics/**`,
+      `!${legacyOutputDir}/router/**/test/**`,
+      `!${legacyOutputDir}/zone.js/**/test/**`,
+      `!${legacyOutputDir}/platform-browser/testing/e2e_util.js`,
+
+    ]
+  );
+}
+
+/**
+ * Queries for all spec files in the built output and creates a single
+ * ESM entry-point file which imports all the spec files.
+ *
+ * This spec file can then be used as entry-point for ESBuild in order
+ * to bundle all specs in an IIFE file.
+ */
+async function createEntryPointSpecFile() {
+  const testFiles = await findSpecFiles();
+
+  let specEntryPointFile = `import './tools/testing/init_browser_spec.ts';`;
+  let i = 0;
+  const testNamespaces = [];
+
+  for (const file of testFiles) {
+    const relativePath = relative(projectDir, file);
+    const specifier = `./${relativePath.replace(/\\/g, '/')}`;
+    const testNamespace = `__${i++}`;
+
+    testNamespaces.push(testNamespace);
+    specEntryPointFile += `import * as ${testNamespace} from '${specifier}';\n`;
+  }
+
+  for (const namespaceId of testNamespaces) {
+    // We generate a side-effect invocation that references the test import. This
+    // is necessary to trick `ESBuild` in preserving the imports. Unfortunately the
+    // test files would be dead-code eliminated otherwise because the specs are part
+    // of folders with `package.json` files setting the `"sideEffects: false"` field.
+    specEntryPointFile += `new Function('x', 'return x')(${namespaceId});\n`;
+  }
+
+  return specEntryPointFile;
+}
+
+/**
+ * Creates an ESBuild plugin which maps `@angular/<..>` module names to their
+ * locally-built output (for the packages which are built as part of this repo).
+ */
+async function createResolveEsbuildPlugin() {
+  const resolveMappings = new Map([
+    [/@angular\//, `${legacyOutputDir}/`],
+    [/^angular-in-memory-web-api$/, join(legacyOutputDir, 'misc/angular-in-memory-web-api')],
+  ]);
+
+  return {
+    name: 'ng-resolve-esbuild', setup: (build) => {
+      build.onResolve({filter: /(@angular\/|angular-in-memory-web-api)/}, async (args) => {
+        const matchedPattern = Array.from(resolveMappings.keys()).find(
+            pattern => args.path.match(pattern));
+
+        if (matchedPattern === undefined) {
+          return undefined;
+        }
+
+        let resolvedPath = args.path.replace(matchedPattern, resolveMappings.get(matchedPattern));
+        let stats = await statGraceful(resolvedPath);
+
+        // If the resolved path points to a directory, resolve the contained `index.js` file
+        if (stats && stats.isDirectory()) {
+          resolvedPath = join(resolvedPath, 'index.js');
+          stats = await statGraceful(resolvedPath);
+        }
+        // If the resolved path does not exist, check with an explicit JS extension.
+        else if (stats === null) {
+          resolvedPath += '.js';
+          stats = await statGraceful(resolvedPath);
+        }
+
+        return stats !== null ? {path: resolvedPath} : undefined;
+      });
+    }
+  }
+}
+
+/**
+ * Creates an ESM bundle for the downlevel decorator transform. The decorator
+ * downlevel transform can then be used later when we compile the sources and tests.
+ *
+ * Note: This layer of indirection needs to exist since we cannot load TS directly
+ * from an ES module. Running ESBuild first allows us to also transpile the TS fast.
+ */
+async function transpileDecoratorDownlevelTransform() {
+  const result = await esbuild.build({
+    bundle: true,
+    sourceRoot: projectDir,
+    platform: 'node',
+    target: 'es2020',
+    format: 'esm',
+    outfile: decoratorDownlevelOutFile,
+    external: ['typescript'],
+    sourcemap: true,
+    entryPoints: [join(containingDir, 'downlevel_decorator_transform.ts')],
+  });
+
+  if (result.errors.length) {
+    throw Error('Could not build decorator downlevel bundle. See errors above.');
+  }
+}
+
+/**
+ * Compiles the project using the TypeScript compiler in order to produce
+ * JS output of the packages and tests.
+ */
+ async function compileProjectWithTsc() {
+  const {legacyCompilationDownlevelDecoratorTransform} = await import(
+      url.pathToFileURL(decoratorDownlevelOutFile));
+  const config = parseTsconfigFile(legacyTsconfigPath, dirname(legacyTsconfigPath));
+  const program = ts.createProgram(config.fileNames, config.options);
+
+  const result = program.emit(undefined, undefined, undefined, undefined, {
+    // We need to downlevel constructor parameters to make ES2015 JIT work. More details
+    // here: https://github.com/angular/angular/pull/37382.
+    before: [legacyCompilationDownlevelDecoratorTransform(program)],
+  });
+
+  const diagnostics = [
+    ...result.diagnostics,
+    ...program.getSyntacticDiagnostics(),
+    ...program.getSemanticDiagnostics()
+  ];
+
+  if (diagnostics.length) {
+    console.error(ts.formatDiagnosticsWithColorAndContext(diagnostics, {
+      getCanonicalFileName: fileName => fileName,
+      getCurrentDirectory: () => program.getCurrentDirectory(),
+      getNewLine: () => '\n',
+    }));
+
+    throw new Error('Compilation failed. See errors above.');
+  }
+
+  console.info('Built packages and specs using TypeScript.');
+  console.info('The constructor parameters have been downleveled.');
+}
+
+function parseTsconfigFile(tsconfigPath, basePath) {
+  const {config} = ts.readConfigFile(tsconfigPath, ts.sys.readFile);
+  const parseConfigHost = {
+    useCaseSensitiveFileNames: ts.sys.useCaseSensitiveFileNames,
+    fileExists: ts.sys.fileExists,
+    readDirectory: ts.sys.readDirectory,
+    readFile: ts.sys.readFile,
+  };
+
+  // Throw if incorrect arguments are passed to this function. Passing relative base paths
+  // results in root directories not being resolved and in later type checking runtime errors.
+  // More details can be found here: https://github.com/microsoft/TypeScript/issues/37731.
+  if (!isAbsolute(basePath)) {
+    throw Error('Unexpected relative base path has been specified.');
+  }
+
+  return ts.parseJsonConfigFileContent(config, parseConfigHost, basePath, {});
+}
+
+
+/**
+ * Retrieves the `fs.Stats` results for the given path gracefully.
+ * If the file does not exist, returns `null`.
+ */
+async function statGraceful(path) {
+  try {
+    return await fs.promises.stat(path);
+  } catch {
+    return null;
+  }
+}
+
+main().catch(e => {
+  console.error(e);
+  process.exitCode = 1;
+});
diff --git a/tools/legacy-saucelabs/downlevel_decorator_transform.ts b/tools/legacy-saucelabs/downlevel_decorator_transform.ts
new file mode 100644
index 00000000000..e675dff608b
--- /dev/null
+++ b/tools/legacy-saucelabs/downlevel_decorator_transform.ts
@@ -0,0 +1,27 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import ts from 'typescript';
+
+import {TypeScriptReflectionHost} from '../../packages/compiler-cli/src/ngtsc/reflection/src/typescript';
+import {getDownlevelDecoratorsTransform} from '../../packages/compiler-cli/src/transformers/downlevel_decorators_transform/index';
+
+/**
+ * Transform for downleveling Angular decorators and Angular-decorated class
+ * constructor parameters for dependency injection.
+ * See https://github.com/angular/angular-cli/pull/14473 for more details.
+ */
+export function legacyCompilationDownlevelDecoratorTransform(program: ts.Program):
+    ts.TransformerFactory<ts.SourceFile> {
+  const typeChecker = program.getTypeChecker();
+  const reflectionHost = new TypeScriptReflectionHost(typeChecker);
+  // Note: `isCore` is set to `true` since we also process the core package.
+  return getDownlevelDecoratorsTransform(
+      typeChecker, reflectionHost, [], /* isCore */ true,
+      /* enableClosureCompiler */ false, /* skipClassDecorators */ true);
+}
diff --git a/tools/legacy-saucelabs/tsconfig.json b/tools/legacy-saucelabs/tsconfig.json
new file mode 100644
index 00000000000..0cb5809262f
--- /dev/null
+++ b/tools/legacy-saucelabs/tsconfig.json
@@ -0,0 +1,10 @@
+{
+    "compilerOptions": {
+        "target": "ES2020",
+        "module": "ES2020",
+        "baseUrl": ".",
+        "esModuleInterop": true,
+        "moduleResolution": "Node",
+        "rootDirs": ["./", "../../"]
+    }
+}
diff --git a/yarn.lock b/yarn.lock
index 8d86c0dd8f3..a60bf2a93b0 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2603,11 +2603,6 @@ abbrev@1:
   resolved "https://registry.yarnpkg.com/abbrev/-/abbrev-1.1.1.tgz#f8f2c887ad10bf67f634f005b6987fed3179aac8"
   integrity sha512-nne9/IiQ/hzIhY6pdDnbBtz7DjPTKrY00P/zvPSm5pOFkl6xuGrGnXn/VtTNNfNtAfZ9/1RtehkszU9qcTii0Q==
 
-abbrev@1.0.x:
-  version "1.0.9"
-  resolved "https://registry.yarnpkg.com/abbrev/-/abbrev-1.0.9.tgz#91b4792588a7738c25f35dd6f63752a2f8776135"
-  integrity sha1-kbR5JYinc4wl813W9jdSovh3YTU=
-
 abort-controller@^3.0.0:
   version "3.0.0"
   resolved "https://registry.yarnpkg.com/abort-controller/-/abort-controller-3.0.0.tgz#eaf54d53b62bae4138e809ca225c8439a6efb392"
@@ -2727,11 +2722,6 @@ ajv@^8.0.0:
     require-from-string "^2.0.2"
     uri-js "^4.2.2"
 
-amdefine@>=0.0.4:
-  version "1.0.1"
-  resolved "https://registry.yarnpkg.com/amdefine/-/amdefine-1.0.1.tgz#4a5282ac164729e93619bcfd3ad151f817ce91f5"
-  integrity sha1-SlKCrBZHKek2Gbz9OtFR+BfOkfU=
-
 "angular-1.5@npm:angular@1.5":
   version "1.5.11"
   resolved "https://registry.yarnpkg.com/angular/-/angular-1.5.11.tgz#8c5ba7386f15965c9acf3429f6881553aada30d6"
@@ -3173,7 +3163,7 @@ async@0.9.x:
   resolved "https://registry.yarnpkg.com/async/-/async-0.9.2.tgz#aea74d5e61c1f899613bf64bda66d4c78f2fd17d"
   integrity sha1-rqdNXmHB+JlhO/ZL2mbUx48v0X0=
 
-async@1.x, async@^1.3.0, async@^1.5.2:
+async@^1.3.0, async@^1.5.2:
   version "1.5.2"
   resolved "https://registry.yarnpkg.com/async/-/async-1.5.2.tgz#ec6a61ae56480c0c3cb241c95618e20892f9672a"
   integrity sha1-7GphrlZIDAw8skHJVhjiCJL5Zyo=
@@ -5893,18 +5883,6 @@ escape-string-regexp@^4.0.0:
   resolved "https://registry.yarnpkg.com/escape-string-regexp/-/escape-string-regexp-4.0.0.tgz#14ba83a5d373e3d311e5afca29cf5bfad965bf34"
   integrity sha512-TtpcNJ3XAzx3Gq8sWRzJaVajRs0uVxA2YAkdb1jm2YkPz4G6egUFAyA3n5vtEIZefPk5Wa4UXbKuS5fKkJWdgA==
 
-escodegen@1.8.x:
-  version "1.8.1"
-  resolved "https://registry.yarnpkg.com/escodegen/-/escodegen-1.8.1.tgz#5a5b53af4693110bebb0867aa3430dd3b70a1018"
-  integrity sha1-WltTr0aTEQvrsIZ6o0MN07cKEBg=
-  dependencies:
-    esprima "^2.7.1"
-    estraverse "^1.9.1"
-    esutils "^2.0.2"
-    optionator "^0.8.1"
-  optionalDependencies:
-    source-map "~0.2.0"
-
 escodegen@^1.8.1:
   version "1.14.3"
   resolved "https://registry.yarnpkg.com/escodegen/-/escodegen-1.14.3.tgz#4e7b81fba61581dc97582ed78cab7f0e8d63f503"
@@ -5942,11 +5920,6 @@ eslint-visitor-keys@^2.0.0:
   resolved "https://registry.yarnpkg.com/eslint-visitor-keys/-/eslint-visitor-keys-2.1.0.tgz#f65328259305927392c938ed44eb0a5c9b2bd303"
   integrity sha512-0rSmRBzXgDzIsD6mGdJgevzgezI534Cer5L/vyMX0kHzT/jiB43jRhd9YUlMGYLQy2zprNmoT8qasCGtY+QaKw==
 
-esprima@2.7.x, esprima@^2.7.1:
-  version "2.7.3"
-  resolved "https://registry.yarnpkg.com/esprima/-/esprima-2.7.3.tgz#96e3b70d5779f6ad49cd032673d1c312767ba581"
-  integrity sha1-luO3DVd59q1JzQMmc9HDEnZ7pYE=
-
 esprima@4.0.1, esprima@^4.0.0, esprima@^4.0.1, esprima@~4.0.0:
   version "4.0.1"
   resolved "https://registry.yarnpkg.com/esprima/-/esprima-4.0.1.tgz#13b04cdb3e6c5d19df91ab6987a8695619b0aa71"
@@ -5959,11 +5932,6 @@ esrecurse@^4.3.0:
   dependencies:
     estraverse "^5.2.0"
 
-estraverse@^1.9.1:
-  version "1.9.3"
-  resolved "https://registry.yarnpkg.com/estraverse/-/estraverse-1.9.3.tgz#af67f2dc922582415950926091a4005d29c9bb44"
-  integrity sha1-r2fy3JIlgkFZUJJgkaQAXSnJu0Q=
-
 estraverse@^4.1.1, estraverse@^4.2.0:
   version "4.3.0"
   resolved "https://registry.yarnpkg.com/estraverse/-/estraverse-4.3.0.tgz#398ad3f3c5a24948be7725e83d11a7de28cdbd1d"
@@ -7119,17 +7087,6 @@ glob@7.2.0, glob@^7.0.0, glob@^7.0.3, glob@^7.0.6, glob@^7.1.1, glob@^7.1.2, glo
     once "^1.3.0"
     path-is-absolute "^1.0.0"
 
-glob@^5.0.15:
-  version "5.0.15"
-  resolved "https://registry.yarnpkg.com/glob/-/glob-5.0.15.tgz#1bc936b9e02f4a603fcc222ecf7633d30b8b93b1"
-  integrity sha1-G8k2ueAvSmA/zCIuz3Yz0wuLk7E=
-  dependencies:
-    inflight "^1.0.4"
-    inherits "2"
-    minimatch "2 || 3"
-    once "^1.3.0"
-    path-is-absolute "^1.0.0"
-
 global-agent@^2.1.12:
   version "2.2.0"
   resolved "https://registry.yarnpkg.com/global-agent/-/global-agent-2.2.0.tgz#566331b0646e6bf79429a16877685c4a1fbf76dc"
@@ -7470,7 +7427,7 @@ handle-thing@^2.0.0:
   resolved "https://registry.yarnpkg.com/handle-thing/-/handle-thing-2.0.1.tgz#857f79ce359580c340d43081cc648970d0bb234e"
   integrity sha512-9Qn4yBxelxoh2Ow62nP+Ka/kMnOXRi8BXnRaUwezLNhqelnN49xKz4F/dPP8OYLxLxq6JDtZb2i9XznUQbNPTg==
 
-handlebars@^4.0.1, handlebars@^4.7.6:
+handlebars@^4.7.6:
   version "4.7.7"
   resolved "https://registry.yarnpkg.com/handlebars/-/handlebars-4.7.7.tgz#9ce33416aad02dbd6c8fafa8240d5d98004945a1"
   integrity sha512-aAcXm5OAfE/8IXkcZvCepKU3VzW1/39Fb5ZuqMtgI/hT8X2YgoMvBY5dLhq/cpOvw7Lk1nK/UF71aLG/ZnVYRA==
@@ -7507,11 +7464,6 @@ has-ansi@^2.0.0:
   dependencies:
     ansi-regex "^2.0.0"
 
-has-flag@^1.0.0:
-  version "1.0.0"
-  resolved "https://registry.yarnpkg.com/has-flag/-/has-flag-1.0.0.tgz#9d9e793165ce017a00f00418c43f942a7b1d11fa"
-  integrity sha1-nZ55MWXOAXoA8AQYxD+UKnsdEfo=
-
 has-flag@^2.0.0:
   version "2.0.0"
   resolved "https://registry.yarnpkg.com/has-flag/-/has-flag-2.0.0.tgz#e8207af1cc7b30d446cc70b734b5e8be18f88d51"
@@ -8573,26 +8525,6 @@ istanbul-reports@^3.0.2:
     html-escaper "^2.0.0"
     istanbul-lib-report "^3.0.0"
 
-istanbul@^0.4.5:
-  version "0.4.5"
-  resolved "https://registry.yarnpkg.com/istanbul/-/istanbul-0.4.5.tgz#65c7d73d4c4da84d4f3ac310b918fb0b8033733b"
-  integrity sha1-ZcfXPUxNqE1POsMQuRj7C4Azczs=
-  dependencies:
-    abbrev "1.0.x"
-    async "1.x"
-    escodegen "1.8.x"
-    esprima "2.7.x"
-    glob "^5.0.15"
-    handlebars "^4.0.1"
-    js-yaml "3.x"
-    mkdirp "0.5.x"
-    nopt "3.x"
-    once "1.x"
-    resolve "1.1.x"
-    supports-color "^3.1.0"
-    which "^1.1.1"
-    wordwrap "^1.0.0"
-
 isurl@^1.0.0-alpha5:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/isurl/-/isurl-1.0.0.tgz#b27f4f49f3cdaa3ea44a0a5b7f3462e6edc39d67"
@@ -8703,7 +8635,7 @@ js-levenshtein@^1.1.6:
   resolved "https://registry.yarnpkg.com/js-tokens/-/js-tokens-4.0.0.tgz#19203fb59991df98e3a287050d4647cdeaf32499"
   integrity sha512-RdJUflcE3cUzKiMqQgsCu06FPu9UdIJO0beYbPhHN4k6apgJtifcoCtT9bcxOpYBtpD2kCM6Sbzg4CausW/PKQ==
 
-js-yaml@3.x, js-yaml@^3.13.1:
+js-yaml@^3.13.1:
   version "3.14.1"
   resolved "https://registry.yarnpkg.com/js-yaml/-/js-yaml-3.14.1.tgz#dae812fdb3825fa306609a8717383c50c36a0537"
   integrity sha512-okMH7OXXJ7YrN9Ok3/SXrnu4iX9yOk+25nqX4imS2npuvTYDmo/QEZoqwZkYaIDk3jVvBOTOIEgEhaLOynBS9g==
@@ -8936,14 +8868,6 @@ karma-jasmine@^4.0.1:
   dependencies:
     jasmine-core "^3.6.0"
 
-karma-parallel@^0.3.1:
-  version "0.3.1"
-  resolved "https://registry.yarnpkg.com/karma-parallel/-/karma-parallel-0.3.1.tgz#781876e5992b2781619e762f3e04dcbf85f47fa1"
-  integrity sha512-64jxNYamYi/9Y67h4+FfViSYhwDgod3rLuq+ZdZ0c3XeZFp/3q3v3HVkd8b5Czp3hCB+LLF8DIv4zlR4xFqbRw==
-  dependencies:
-    istanbul "^0.4.5"
-    lodash "^4.17.11"
-
 karma-requirejs@^1.1.0:
   version "1.1.0"
   resolved "https://registry.yarnpkg.com/karma-requirejs/-/karma-requirejs-1.1.0.tgz#fddae2cb87d7ebc16fb0222893564d7fee578798"
@@ -9858,7 +9782,7 @@ minimalistic-assert@^1.0.0, minimalistic-assert@^1.0.1:
   resolved "https://registry.yarnpkg.com/minimalistic-assert/-/minimalistic-assert-1.0.1.tgz#2e194de044626d4a10e7f7fbc00ce73e83e4d5c7"
   integrity sha512-UtJcAD4yEaGtjPezWuO9wC4nwUnVH/8/Im3yEHQP4b67cXlD/Qr9hdITCU1xDbSEXg2XKNaP8jsReV7vQd00/A==
 
-"minimatch@2 || 3", minimatch@3.0.4, minimatch@^3.0.3, minimatch@^3.0.4:
+minimatch@3.0.4, minimatch@^3.0.3, minimatch@^3.0.4:
   version "3.0.4"
   resolved "https://registry.yarnpkg.com/minimatch/-/minimatch-3.0.4.tgz#5166e286457f03306064be5497e8dbb0c3d32083"
   integrity sha512-yJHVQEhyqPLUTgt9B83PXu6W3rx4MvvHvSUvToogpwoGDOUQ+yDrR0HRot+yOCdCO7u4hX3pWft6kWBBcqh0UA==
@@ -9969,7 +9893,7 @@ mkdirp-classic@^0.5.2:
   resolved "https://registry.yarnpkg.com/mkdirp-classic/-/mkdirp-classic-0.5.3.tgz#fa10c9115cc6d8865be221ba47ee9bed78601113"
   integrity sha512-gKLcREMhtuZRwRAfqP3RFW+TK4JqApVBtOIftVgjuABpAtpxhPGaDcfvbhNvD0B8iD1oUr/txX35NjcaY6Ns/A==
 
-mkdirp@0.5.x, "mkdirp@>=0.5 0", mkdirp@^0.5.1, mkdirp@^0.5.3, mkdirp@^0.5.5:
+"mkdirp@>=0.5 0", mkdirp@^0.5.1, mkdirp@^0.5.3, mkdirp@^0.5.5:
   version "0.5.5"
   resolved "https://registry.yarnpkg.com/mkdirp/-/mkdirp-0.5.5.tgz#d91cefd62d1436ca0f41620e251288d420099def"
   integrity sha512-NKmAlESf6jMGym1++R0Ra7wvhV+wFW63FaSOFPwRahvea0gMUcGUhVeAg/0BC0wiv9ih5NYPB1Wn1UEI1/L+xQ==
@@ -10049,7 +9973,7 @@ multicast-dns@^6.0.1:
     dns-packet "^1.3.1"
     thunky "^1.0.2"
 
-multimatch@*:
+multimatch@*, multimatch@^6.0.0:
   version "6.0.0"
   resolved "https://registry.yarnpkg.com/multimatch/-/multimatch-6.0.0.tgz#c72a9bddbc94baa4727efd613b5d22a1fe4d6ee3"
   integrity sha512-I7tSVxHGPlmPN/enE3mS1aOSo6bWBfls+3HmuEeCUBCE7gWnm3cBXCBkpurzFjVRwC6Kld8lLaZ1Iv5vOcjvcQ==
@@ -10254,13 +10178,6 @@ nodejs-websocket@^1.7.2:
   resolved "https://registry.yarnpkg.com/nodejs-websocket/-/nodejs-websocket-1.7.2.tgz#94abd1e248f57d4d1c663dec3831015c6dad98a6"
   integrity sha512-PFX6ypJcCNDs7obRellR0DGTebfUhw1SXGKe2zpB+Ng1DQJhdzbzx1ob+AvJCLzy2TJF4r8cCDqMQqei1CZdPQ==
 
-nopt@3.x:
-  version "3.0.6"
-  resolved "https://registry.yarnpkg.com/nopt/-/nopt-3.0.6.tgz#c6465dbf08abcd4db359317f79ac68a646b28ff9"
-  integrity sha1-xkZdvwirzU2zWTF/eaxopkayj/k=
-  dependencies:
-    abbrev "1"
-
 nopt@^5.0.0:
   version "5.0.0"
   resolved "https://registry.yarnpkg.com/nopt/-/nopt-5.0.0.tgz#530942bb58a512fccafe53fe210f13a25355dc88"
@@ -10549,7 +10466,7 @@ on-headers@^1.0.0, on-headers@~1.0.2:
   resolved "https://registry.yarnpkg.com/on-headers/-/on-headers-1.0.2.tgz#772b0ae6aaa525c399e489adfad90c403eb3c28f"
   integrity sha512-pZAE+FJLoyITytdqK0U5s+FIpjN0JP3OzFi/u8Rx+EV5/W+JTWGXG8xFzevE7AjBfDqHv/8vL8qQsIhHnqRkrA==
 
-once@1.x, once@^1.3.0, once@^1.3.1, once@^1.3.2, once@^1.4.0:
+once@^1.3.0, once@^1.3.1, once@^1.3.2, once@^1.4.0:
   version "1.4.0"
   resolved "https://registry.yarnpkg.com/once/-/once-1.4.0.tgz#583b1aa775961d4b113ac17d9c50baef9dd76bd1"
   integrity sha1-WDsap3WWHUsROsF9nFC6753Xa9E=
@@ -12362,11 +12279,6 @@ resolve-url@^0.2.1:
   resolved "https://registry.yarnpkg.com/resolve-url/-/resolve-url-0.2.1.tgz#2c637fe77c893afd2a663fe21aa9080068e2052a"
   integrity sha1-LGN/53yJOv0qZj/iGqkIAGjiBSo=
 
-resolve@1.1.x:
-  version "1.1.7"
-  resolved "https://registry.yarnpkg.com/resolve/-/resolve-1.1.7.tgz#203114d82ad2c5ed9e8e0411b3932875e889e97b"
-  integrity sha1-IDEU2CrSxe2ejgQRs5ModeiJ6Xs=
-
 resolve@1.20.0, resolve@^1.1.6, resolve@^1.1.7, resolve@^1.10.0, resolve@^1.14.2, resolve@^1.17.0, resolve@^1.19.0, resolve@^1.3.2, resolve@^1.4.0:
   version "1.20.0"
   resolved "https://registry.yarnpkg.com/resolve/-/resolve-1.20.0.tgz#629a013fb3f70755d6f0b7935cc1c2c5378b1975"
@@ -12644,7 +12556,7 @@ sass@1.43.4:
 
 "sauce-connect@https://saucelabs.com/downloads/sc-4.7.1-linux.tar.gz":
   version "0.0.0"
-  resolved "https://saucelabs.com/downloads/sc-4.7.1-linux.tar.gz#e5d7f82ad98251a653d1b0537f1103e49eda5e11"
+  resolved "https://saucelabs.com/downloads/sc-4.6.2-linux.tar.gz#7b7f35433af9c3380758e048894d7b9aecf3754e"
 
 saucelabs@4.7.8, saucelabs@^1.5.0, saucelabs@^4.6.3:
   version "4.7.8"
@@ -13201,13 +13113,6 @@ source-map@^0.5.0, source-map@^0.5.1, source-map@^0.5.6:
   resolved "https://registry.yarnpkg.com/source-map/-/source-map-0.5.7.tgz#8a039d2d1021d22d1ea14c80d8ea468ba2ef3fcc"
   integrity sha1-igOdLRAh0i0eoUyA2OpGi6LvP8w=
 
-source-map@~0.2.0:
-  version "0.2.0"
-  resolved "https://registry.yarnpkg.com/source-map/-/source-map-0.2.0.tgz#dab73fbcfc2ba819b4de03bd6f6eaa48164b3f9d"
-  integrity sha1-2rc/vPwrqBm03gO9b26qSBZLP50=
-  dependencies:
-    amdefine ">=0.0.4"
-
 sourcemap-codec@1.4.8, sourcemap-codec@^1.4.4, sourcemap-codec@^1.4.8:
   version "1.4.8"
   resolved "https://registry.yarnpkg.com/sourcemap-codec/-/sourcemap-codec-1.4.8.tgz#ea804bd94857402e6992d05a38ef1ae35a9ab4c4"
@@ -13584,13 +13489,6 @@ supports-color@^2.0.0:
   resolved "https://registry.yarnpkg.com/supports-color/-/supports-color-2.0.0.tgz#535d045ce6b6363fa40117084629995e9df324c7"
   integrity sha1-U10EXOa2Nj+kARcIRimZXp3zJMc=
 
-supports-color@^3.1.0:
-  version "3.2.3"
-  resolved "https://registry.yarnpkg.com/supports-color/-/supports-color-3.2.3.tgz#65ac0504b3954171d8a64946b2ae3cbb8a5f54f6"
-  integrity sha1-ZawFBLOVQXHYpklGsq48u4pfVPY=
-  dependencies:
-    has-flag "^1.0.0"
-
 supports-color@^5.0.0, supports-color@^5.3.0:
   version "5.5.0"
   resolved "https://registry.yarnpkg.com/supports-color/-/supports-color-5.5.0.tgz#e2e69a44ac8772f78a1ec0b35b689df6530efc8f"
@@ -14900,7 +14798,7 @@ which-module@^2.0.0:
   resolved "https://registry.yarnpkg.com/which-module/-/which-module-2.0.0.tgz#d9ef07dce77b9902b8a3a8fa4b31c3e3f7e6e87a"
   integrity sha1-2e8H3Od7mQK4o6j6SzHD4/fm6Ho=
 
-which@^1.1.1, which@^1.2.1, which@^1.2.14, which@^1.2.9:
+which@^1.2.1, which@^1.2.14, which@^1.2.9:
   version "1.3.1"
   resolved "https://registry.yarnpkg.com/which/-/which-1.3.1.tgz#a45043d54f5805316da8d62f9f50918d3da70b0a"
   integrity sha512-HxJdYWq1MTIQbJ3nw0cqssHoTNU267KlrDuGZ1WYlxDStUtKUhOaJmh112/TZmHxxUfuJqPXSOm7tDyas0OSIQ==