chore: run a11y audits on protractor (#2010)

* chore: run a11y audits on protractor

* Fixes accessibility issue in grid-list with `role="listitem"`
* Adds protractor plugin to run aXe-core accessibility audits
* Fixes a bunch of a11y issues in the e2e app to make the aXe audits green

* Force menu y-positions because viewport might be different on Selenium browsers.

* Navigiation links should be hidden by default because those can interfere with the tests.

test/protractor.conf.js

@@ -9,7 +9,6 @@ require('ts-node').register({
 
 const E2E_BASE_URL = process.env['E2E_BASE_URL'] || 'http://localhost:4200';
 const config = {
-  // TODO(jelbourn): add back plugin for a11y assersions once it supports specifying AXS options.
   useAllAngular2AppRoots: true,
   specs: [ path.join(__dirname, '../e2e/**/*.e2e.ts') ],
   baseUrl: E2E_BASE_URL,
@@ -17,7 +16,20 @@ const config = {
   getPageTimeout: 120000,
   jasmineNodeOpts: {
     defaultTimeoutInterval: 120000,
-  }
+  },
+
+  plugins: [
+    {
+      // Runs the axe-core accessibility checks each time the e2e page changes and
+      // Angular is ready.
+      path: '../tools/axe-protractor/axe-protractor.js',
+
+      rules: [
+        // Exclude md-menu elements because those are empty if not active.
+        { id: 'aria-required-children', selector: '*:not(md-menu)' },
+      ]
+    }
+  ]
 };
 
 if (process.env['TRAVIS']) {