fix(@angular-devkit/build-angular): don't rerun tests on unchanged compilation

Fix angular#11880

Author: filipesilva

packages/angular_devkit/build_angular/src/angular-cli-files/plugins/karma.ts

@@ -174,9 +174,11 @@ const init: any = (config: any, emitter: any, customFileHandlers: any) => {
     blocked = [];
   }
 
+  let lastCompilationHash: string | undefined;
   compiler.hooks.done.tap('karma', (stats: any) => {
-    // Don't refresh karma when there are webpack errors.
-    if (stats.compilation.errors.length === 0) {
+    // Refresh karma only when there are no webpack errors, and if the compilation changed.
+    if (stats.compilation.errors.length === 0 && stats.hash != lastCompilationHash) {
+      lastCompilationHash = stats.hash;
       emitter.refreshFiles();
     }
     unblock();

packages/angular_devkit/build_angular/test/karma/rebuilds_spec_large.ts

@@ -13,7 +13,8 @@ import { host, karmaTargetSpec } from '../utils';
 
 // Karma watch mode is currently bugged:
 // - errors print a huge stack trace
-// - karma does not have a way to close the server gracefully.
+// - karma does not have a way to close the server
+//   gracefully (https://github.com/karma-runner/karma-jasmine/issues/219)
 // TODO: fix these before 6.0 final.
 xdescribe('Karma Builder watch mode', () => {
   beforeEach(done => host.initialize().toPromise().then(done, done.fail));
@@ -64,4 +65,9 @@ xdescribe('Karma Builder watch mode', () => {
       take(3),
     ).toPromise().then(done, done.fail);
   }, 30000);
+
+  it('does not rebuild when nothing changed', (done) => {
+    // Start the server in watch mode, wait for the first build to finish, touch
+    // test.js without changing it, wait 5s then exit unsuscribe, verify only one event was emitted.
+  }, 30000);
 });