fix(core): remove NgZone_ and use NgZone instead

modules/angular2/test/core/render/dom/events/event_manager_spec.ts

@@ -14,7 +14,7 @@ import {
   EventManagerPlugin,
   DomEventsPlugin
 } from 'angular2/src/core/render/dom/events/event_manager';
-import {NgZone, NgZone_} from 'angular2/src/core/zone/ng_zone';
+import {NgZone} from 'angular2/src/core/zone/ng_zone';
 import {ListWrapper, Map, MapWrapper} from 'angular2/src/core/facade/collection';
 import {DOM} from 'angular2/src/core/dom/dom_adapter';
 
@@ -106,7 +106,7 @@ class FakeEventManagerPlugin extends EventManagerPlugin {
   }
 }
 
-class FakeNgZone extends NgZone_ {
+class FakeNgZone extends NgZone {
   constructor() { super({enableLongStackTrace: false}); }
   run(fn) { fn(); }