fix(packages): use ES modules for primary build (#11120)

modules/@angular/platform-browser/testing/index.ts

@@ -0,0 +1,14 @@
+/**
+ * @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
+ */
+
+/**
+ * @module
+ * @description
+ * Entry point for all public APIs of the platform-browser/testing package.
+ */
+export * from './browser';