test(compiler-cli): improve compliance test performance (#39956)

The newly built compliance test runner was not using the shared source
file cache that was added in b627f7f02e,
which offers a significant performance boost to the compliance test
targets.

PR Close #39956

packages/compiler-cli/src/ngtsc/testing/index.ts

@@ -7,5 +7,6 @@
  */
 export * from './src/utils';
 export * from './src/cached_source_files';
+export * from './src/compiler_host';
 export * from './src/mock_file_loading';
 export * from './src/runfile_helpers';

packages/compiler-cli/src/ngtsc/testing/src/compiler_host.ts

@@ -0,0 +1,25 @@
+/**
+ * @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 * as ts from 'typescript';
+import {NgtscCompilerHost} from '../../file_system';
+import {getCachedSourceFile} from './cached_source_files';
+
+/**
+ * A compiler host intended to improve test performance by caching default library source files for
+ * reuse across tests.
+ */
+export class NgtscTestCompilerHost extends NgtscCompilerHost {
+  getSourceFile(fileName: string, languageVersion: ts.ScriptTarget): ts.SourceFile|undefined {
+    const cachedSf = getCachedSourceFile(fileName, () => this.readFile(fileName));
+    if (cachedSf !== null) {
+      return cachedSf;
+    }
+    return super.getSourceFile(fileName, languageVersion);
+  }
+}