fix(build): Use Angular's testability API to wait for end of e2e tests

Closes #3911

modules/angular2/src/core/testability/browser_testability.dart

@@ -63,6 +63,10 @@ class PublicTestability implements _JsObjectProxyable {
     this._testability = testability;
   }
 
+  bool isStable() {
+    return this._testability.isStable();
+  }
+
   whenStable(Function callback) {
     return this._testability.whenStable(callback);
   }
@@ -75,7 +79,8 @@ class PublicTestability implements _JsObjectProxyable {
     return _jsify({
       'findBindings': (bindingString, [exactMatch, allowNonElementNodes]) =>
           findBindings(bindingString, exactMatch, allowNonElementNodes),
-      'whenStable': (callback) => whenStable(() => callback.apply([])),
+      'isStable': () => isStable(),
+      'whenStable': (callback) => whenStable(() => callback.apply([]))
     })..['_dart_'] = this;
   }
 }

modules/angular2/src/core/testability/browser_testability.ts

@@ -11,6 +11,8 @@ class PublicTestability {
 
   constructor(testability: Testability) { this._testability = testability; }
 
+  isStable(): boolean { return this._testability.isStable(); }
+
   whenStable(callback: Function) { this._testability.whenStable(callback); }
 
   findBindings(using: any, binding: string, exactMatch: boolean): any[] {

modules/angular2/src/core/testability/testability.ts

@@ -42,8 +42,10 @@ export class Testability {
     return this._pendingCount;
   }
 
+  isStable(): boolean { return this._pendingCount == 0 && !this._isAngularEventPending; }
+
   _runCallbacksIfReady(): void {
-    if (this._pendingCount != 0 || this._isAngularEventPending) {
+    if (!this.isStable()) {
       return;  // Not ready
     }