chore(Test): Start making the Dart2JS golden file test more useful:

* Make it serve-able (index.html)
* Actually make it use AngularDart

PiperOrigin-RevId: 200726171

Author: matanlurey

_goldens/test/_files/dart2js/dart2js_golden.dart

@@ -1,3 +1,7 @@
+import 'package:angular/angular.dart';
+
+import 'dart2js_golden.template.dart' as ng;
+
 /// This file entirely exists as synthetic AngularDart application.
 ///
 /// The results, compiled with Dart2JS (without minification) are expected to
@@ -8,5 +12,11 @@
 ///
 /// **NOTE**: The test is not executed externally.
 void main() {
-  print('Hello World');
+  runApp(ng.RootComponentNgFactory);
 }
+
+@Component(
+  selector: 'root-component',
+  template: '',
+)
+class RootComponent {}