const expectedHeroes = [{name: 'A'}, {name: 'B'}]
const mockService =  {getHeroes: () => expectedHeroes }

it('should have heroes when HeroListComponent created', () => {
  // Pass the mock to the constructor as the Angular injector would
  const component = new HeroListComponent(mockService);
  expect(component.heroes.length).toEqual(expectedHeroes.length);
});