refactor(examples): update the Zippy example to rely on ambient directives

modules/playground/pubspec.yaml

@@ -17,6 +17,7 @@ dependency_overrides:
     path: ../angular2_material
 transformers:
 - angular2:
+    ambient_directives: 'angular2/lib/src/core/directives.dart:CORE_DIRECTIVES'
     entry_points:
         - web/src/gestures/index.dart
         - web/src/hello_world/index.dart

modules/playground/src/zippy_component/index.ts

@@ -1,5 +1,5 @@
 import {bootstrap} from 'angular2/bootstrap';
-import {Component, View, NgFor} from 'angular2/core';
+import {Component, View} from 'angular2/core';
 import {Zippy} from './zippy';
 
 @Component({selector: 'zippy-app'})
@@ -12,7 +12,7 @@ import {Zippy} from './zippy';
       <li *ng-for="var log of logs">{{log}}</li>
     </ul>
   `,
-  directives: [Zippy, NgFor]
+  directives: [Zippy]
 })
 class ZippyApp {
   logs: string[] = [];