anchor.diff:      我们遵循的文件命名约定是：服务名称的小写形式（基本名），加上`.service`后缀。
       如果服务名称包含多个单词，我们就把基本名部分写成中线形式 ([dash-case](../guide/glossary.html#dash-case))。
       例如，`SpecialSuperHeroService`服务应该被定义在`special-super-hero.service.ts`文件中。
  +    
   :marked
     Name the class `HeroService` and export it for others to import.
   
  
positive.code:   把`HeroService`的`getHeroes`方法改写为返回承诺的形式：
    
  +makeExample('toh-4/ts/src/app/hero.service.ts', 'get-heroes', 'src/app/hero.service.ts (excerpt)')(format=".")
  :marked
    You're still mocking the data. You're simulating the behavior of an ultra-fast, zero-latency server,
    by returning an *immediately resolved Promise* with the mock heroes as the result.
  
negative.code:   Notice that you imported the Angular `Injectable` function and applied that function as an `@Injectable()` decorator.
  
    注意，我们导入了 Angular 的`Injectable`函数，并作为`@Injectable()`装饰器使用这个函数。
    
  .callout.is-helpful
    :marked
      Don't forget the parentheses. Omitting them leads to an error that's difficult to diagnose.
  
      **不要忘了写圆括号！**如果忘了写，就会导致一个很难诊断的错误。
      
  :marked
    The `@Injectable()` decorator tells TypeScript to emit metadata about the service.
    The metadata specifies that Angular may need to inject other dependencies into this service.
  