fix(@angular-devkit/build-angular): watch index file when running build in watch mode

Since the index augmentation happens outside of Webpack previously the index html was not being watched during watch mode.

Closes #23851

(cherry picked from commit 130975c)

Author: alan-agius4

packages/angular_devkit/build_angular/src/builders/browser/index.ts

@@ -173,6 +173,19 @@ export function buildWebpackBrowser(
       // Check and warn about IE browser support
       checkInternetExplorerSupport(initialization.projectRoot, context.logger);
 
+      // Add index file to watched files.
+      if (options.watch) {
+        const indexInputFile = path.join(context.workspaceRoot, getIndexInputFile(options.index));
+        initialization.config.plugins ??= [];
+        initialization.config.plugins.push({
+          apply: (compiler: webpack.Compiler) => {
+            compiler.hooks.thisCompilation.tap('build-angular', (compilation) => {
+              compilation.fileDependencies.add(indexInputFile);
+            });
+          },
+        });
+      }
+
       return {
         ...initialization,
         cacheOptions: normalizeCacheOptions(projectMetadata, context.workspaceRoot),

packages/angular_devkit/build_angular/src/builders/browser/tests/behavior/index_watch_spec.ts

@@ -0,0 +1,53 @@
+/**
+ * @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 { concatMap, count, take, timeout } from 'rxjs/operators';
+import { BUILD_TIMEOUT, buildWebpackBrowser } from '../../index';
+import { BASE_OPTIONS, BROWSER_BUILDER_INFO, describeBuilder } from '../setup';
+
+describeBuilder(buildWebpackBrowser, BROWSER_BUILDER_INFO, (harness) => {
+  describe('Behavior: "index is updated during watch mode"', () => {
+    it('index is watched in watch mode', async () => {
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        watch: true,
+      });
+
+      const buildCount = await harness
+        .execute()
+        .pipe(
+          timeout(BUILD_TIMEOUT),
+          concatMap(async ({ result }, index) => {
+            expect(result?.success).toBe(true);
+
+            switch (index) {
+              case 0: {
+                harness.expectFile('dist/index.html').content.toContain('HelloWorldApp');
+                harness.expectFile('dist/index.html').content.not.toContain('UpdatedPageTitle');
+
+                // Trigger rebuild
+                await harness.modifyFile('src/index.html', (s) =>
+                  s.replace('HelloWorldApp', 'UpdatedPageTitle'),
+                );
+                break;
+              }
+              case 1: {
+                harness.expectFile('dist/index.html').content.toContain('UpdatedPageTitle');
+                break;
+              }
+            }
+          }),
+          take(2),
+          count(),
+        )
+        .toPromise();
+
+      expect(buildCount).toBe(2);
+    });
+  });
+});