test(@angular-devkit/build-angular): add browser builder polyfills option tests

This change adds expanded unit tests for the browser builder's `polyfills` option using the builder test harness.

packages/angular_devkit/build_angular/src/browser/tests/options/polyfills_spec.ts

@@ -0,0 +1,57 @@
+/**
+ * @license
+ * Copyright Google Inc. 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 { buildWebpackBrowser } from '../../index';
+import { BASE_OPTIONS, BROWSER_BUILDER_INFO, describeBuilder } from '../setup';
+
+describeBuilder(buildWebpackBrowser, BROWSER_BUILDER_INFO, (harness) => {
+  describe('Option: "polyfills"', () => {
+    it('uses a provided TypeScript file', async () => {
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        polyfills: 'src/polyfills.ts',
+      });
+
+      const { result } = await harness.executeOnce();
+
+      expect(result?.success).toBe(true);
+
+      harness.expectFile('dist/polyfills.js').toExist();
+    });
+
+    it('uses a provided JavaScript file', async () => {
+      await harness.writeFile('src/polyfills.js', `console.log('main');`);
+
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        polyfills: 'src/polyfills.js',
+      });
+
+      const { result } = await harness.executeOnce();
+
+      expect(result?.success).toBe(true);
+
+      harness.expectFile('dist/polyfills.js').content.toContain(`console.log('main')`);
+    });
+
+    it('fails and shows an error when file does not exist', async () => {
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        polyfills: 'src/missing.ts',
+      });
+
+      const { result, logs } = await harness.executeOnce({ outputLogsOnFailure: false });
+
+      expect(result?.success).toBe(false);
+      expect(logs).toContain(
+        jasmine.objectContaining({ message: jasmine.stringMatching('Module not found:') }),
+      );
+
+      harness.expectFile('dist/polyfills.js').toNotExist();
+    });
+  });
+});