fix 4 issues.

1. proxyZone should call onHasTask when change delegate
2. AsyncTestZoneSpec should only call finishCallback when the it is pure async.
3. jasmine internal setTimeout should not in zone.
4. AsyncTestZoneSpec doesn't handle eventTask correctly.

fix angular/angular#22448, which can be described as the following code. (this issue will also bring some other issues), I have created several cases here, https://github.com/angular/angular/pull/22454/files.

I also created test cases in zone.js to simulate @angular/core/async.

beforeEach(async(() => {
  TestBed.configureTestingModule({
      declarations: [
        AppComponent
      ],
    }).compileComponents();
}));
it('simple timeout', async(() => {
    setTimeout(() => {
      expect(true).toBe(true);
    }, 200);
  }));

the case above will report Error: 'expect' was used when there was no current spec.
Because simple timeout test will not wait for setTimeout finished.

There are several reasons.

1. ProxyZone should call onHasTask when change delegate, otherwise the new delegateZoneSpec may never get onHasTask triggered.

For example:

• we have a proxyZone with delegate zoneSpec1.
• we setTimeout in zoneSpec1
• and we change the delegate to zoneSpec2.
• we setTimeout again

The onHasTask will not triggered in zoneSpec2 when call setTimeout because the taskCount is already be 1 which was increased by zoneSpec1.

2. when we fix the first issue, the case will still fail, because jasmine will call setTimeout to monitor timeout, and this setTimeout will also run in Proxy(AsyncTest)Zone, so there will always a pendingMacroTask to make the test case timeout forever.

3. When we fix the above 2 issues, there will come out a new problem, https://github.com/angular/zone.js/blob/master/lib/zone-spec/async-test.ts#L84, the finishCallback will triggered twice (one by onHasTask, one by onInvoke). We should only trigger the onInvoke one when the callback is pure sync.

4. When we fix all those 3 problems, there still one left in angular side, https://github.com/angular/angular/blob/master/packages/core/testing/src/async.ts#L91, async should reset the delegateSpec outside of currentZone.run.

I have created several test cases with async of @angular/core, and I will also create test cases in zone.js side.

@mhevery , please review this one, it seems will bring some impact of AsyncTestZone and async in angular, thank you!