fix(fake_async): share zone between beforeEach and it

This is needed for the case if a `beforeEach` instantiates
`NgZone`and the `it` uses
`TestComponentBuilder.createFakeAsync`.

Otherwise the `NgZone` will use the root zone as parent,
and `TestComponentBuilder.createFakeAsync` will always return
undefined as `tick` does not forward promises created
under the zone of `NgZone`.

modules/@angular/core/testing/testing.ts

@@ -14,15 +14,19 @@
 
 import {SchemaMetadata} from '../index';
 
+import {resetFakeAsyncZone} from './fake_async';
 import {TestBed, TestModuleMetadata, getTestBed} from './test_bed';
 
 declare var global: any;
 
 var _global = <any>(typeof window === 'undefined' ? global : window);
 
-// Reset the test providers before each test.
+// Reset the test providers and the fake async zone before each test.
 if (_global.beforeEach) {
-  _global.beforeEach(() => { TestBed.resetTestingModule(); });
+  _global.beforeEach(() => {
+    TestBed.resetTestingModule();
+    resetFakeAsyncZone();
+  });
 }
 
 /**