Angular 2测试：在jasmine.DEFAULT_TIMEOUT_INTERVAL指定的超时

Error: Timeout – Async callback was not invoked within timeout specified by jasmine.DEFAULT_TIMEOUT_INTERVAL.

被测组件是：(sorry,it is bulky)

考试：

import { TestBed,ComponentFixture,async,inject } from '@angular/core/testing';
import { DebugElement } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import {Injectable} from '@angular/core';

import {FormGroup,FormBuilder,ReactiveFormsModule} from '@angular/forms';

import {Observable} from 'rxjs/Observable';
import 'rxjs/add/observable/of';

import SignupComponent from './signup.component';
import {UserService} from '../services/user.service';


@Injectable()
export class MockUserService {
    public signup(user: any) {
        return Observable.of({});
    }
}

let component: SignupComponent;
let fixture: ComponentFixture<SignupComponent>;

describe('SignupComponent',() => {
     beforeEach(async(() => {
         TestBed.configureTestingModule({
            declarations: [ SignupComponent ],imports: [
                 BrowserModule,ReactiveFormsModule
            ]
        })
        .overrideComponent(SignupComponent,{
            set: {
                templateUrl: 'app/components/signup.component.html'
            }}
        )
        .overrideComponent(SignupComponent,{
            set: {
                providers: [
                    { provide: UserService,useClass: MockUserService },]
            }
        })
        .compileComponents().then(createComponent);
}));

    it('should create an instance',() => {
        expect(component).toBeDefined();
    });
});

 /***** HELPERS *****/
 function createComponent() {
      fixture = TestBed.createComponent(SignupComponent);
      component = fixture.componentInstance;
      fixture.detectChanges();

     return fixture.whenStable().then(() => {
         fixture.detectChanges();
     });
 }