anchor.diff:  .callout.is-critical
     header Always include the parentheses
     header 总要带着括号
  +  
     :marked
       Always use `@Injectable()`, not just `@Injectable`.
       Our application will fail mysteriously if we forget the parentheses.
  
positive.code: :marked
    ## Creating and registering a logger service
    ## 创建和注册日志服务
    We're injecting a logger into our `HeroService` in two steps:
    
    要把日志服务注入到`HeroService`中需要两步：
    1. Create the logger service.
    1. 创建日志服务。
    1. Register it with the application.
    1. 把它注册到应用中。
  
    The logger service implementation is no big deal.
  
negative.code:   But its child `HeroListComponent` does, so we head there next.
    
    `HeroesComponent`本身不需要`HeroService`，但它的子组件`HeroListComponent`需要，所以我们再往下看。
    
  // #enddocregion di-register-providers-3
  // #docregion di-prepare-for-injection-1
  :marked
  