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

modules/@angular/core/rollup-testing.config.js

@@ -0,0 +1,15 @@
+
+export default {
+  entry: '../../../dist/packages-dist/core/testing/index.js',
+  dest: '../../../dist/packages-dist/core/bundles/core-testing.umd.js',
+  format: 'umd',
+  moduleName: 'ng.core.testing',
+  globals: {
+    '@angular/core': 'ng.core',
+    'rxjs/Subject': 'Rx',
+    'rxjs/observable/PromiseObservable': 'Rx', // this is wrong, but this stuff has changed in rxjs b.6 so we need to fix it when we update.
+    'rxjs/operator/toPromise': 'Rx.Observable.prototype',
+    'rxjs/Observable': 'Rx'
+  }
+}
+