anchor.diff:      Read more about dependency injection in the [Dependency Injection](../guide/dependency-injection.html) page.
   
       更多依赖注入的信息，见[依赖注入](../guide/dependency-injection.html)。
  +    
   :marked
     The *injector* doesn't know yet how to create a `HeroService`.
     If you ran the code now, Angular would fail with this error:
  
positive.code:     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.
  
  
negative.code:   添加构造函数：
    
  +makeExample('toh-4/ts/src/app/app.component.1.ts', 'ctor', 'src/app/app.component.ts (constructor)')
  
  :marked
    The constructor itself does nothing. The parameter simultaneously
    defines a private `heroService` property and identifies it as a `HeroService` injection site.
  