it('should be stable for component OnInit', async(() => {
    const fixture = TestBed.createComponent(DemoComponent);
    fixture.componentInstance.ngOnInit();
    expect(fixture.isStable()).toBe(true);
}));

it('should be unstable for fixture OnInit', async(() => {
    const fixture = TestBed.createComponent(DemoComponent);
    fixture.detectChanges();
    expect(fixture.isStable()).toBe(false);
}));