chore: add test to prevent accidental changes to public API

modules/angular2/src/core/application.ts

@@ -1,4 +1,4 @@
 // Public API for Application
 export {APP_COMPONENT} from './application_tokens';
 export {platform, commonBootstrap as bootstrap} from './application_common';
-export {PlatformRef, ApplicationRef, rootBindings} from './application_ref';
+export {PlatformRef, ApplicationRef} from './application_ref';

modules/angular2/test/public_api_spec.dart

@@ -0,0 +1,2 @@
+// ignore in dart
+main() {}