anchor.diff:    Like `getHeroes`, it also returns a !{_Promise}. 
     But this !{_Promise} waits 2 seconds before resolving the !{_Promise} with mock heroes.
     
  -  像`getHeroes`一样，它也返回一个Promise。
  -  但是，这个Promise会在提供模拟数据之前等待两秒钟。
  +  像`getHeroes`一样，它也返回一个承诺。
  +  但是，这个承诺会在提供模拟数据之前等待两秒钟。
     
     Back in the `AppComponent`, replace `heroService.getHeroes` with `heroService.getHeroesSlowly`
     and see how the app behaves.
  
positive.code: :marked
    Update the `HeroService` with this !{_Promise}-returning `getHeroes` method:
    
    把`HeroService`的`getHeroes`方法改写为返回Promise的形式：
    
  +makeExample('toh-4/ts/app/hero.service.ts', 'get-heroes', 'app/hero.service.ts (excerpt)')(format=".")
  
  
negative.code:   
    ## Async Services and !{_Promise}s
    
    ## 异步服务与承诺
    
    Our `HeroService` returns a list of mock heroes immediately. 
    Its `getHeroes` signature is synchronous
  