anchor.diff:    
     This is a golden rule: **always delegate data access to a supporting service class**.
     
  -  这是一条“黄金法则”：**总是把数据访问工作委托给一个提供支持的服务类**。
  +  这是一条“黄金法则”：**总是把数据访问工作委托给一个支持服务类**。
   
     Although _at runtime_ the component requests heroes immediately after creation, 
     we do **not** call the service's `get` method in the component's constructor.
     We call it inside the `ngOnInit` [lifecycle hook](lifecycle-hooks.html) instead
     and count on Angular to call `ngOnInit` when it instantiates this component. 
     
  -  虽然_在运行期间_，组件会在创建之后立刻请求这些英雄数据，但我们**不会**在组件的构造函数中调用此服务的`get`方法。
  +  虽然_在运行期间_，组件会在创建之后立刻请求这些英雄数据，但我们**不**在组件的构造函数中调用此服务的`get`方法。
     而是在`ngOnInit`[生命周期钩子](lifecycle-hooks.html)中调用它，Angular会在初始化该组件时调用`ngOnInit`方法。
   .l-sub-section
     :marked
  
positive.code:     We should include only those features that we actually need.
      
      RxJS库实在是太大了。
      当我们构建一个产品级应用，并且把它发布到移动设备上的时候，大小就会成为一个问题。
      我们应该只包含哪些我们确实需要的特性。
  
      Accordingly, Angular exposes a stripped down version of `Observable` in the `rxjs/Observable` module, 
      a version that lacks most of the operators including some we'd like to use here
  
negative.code:     We haven't discussed so far how that actually works. 
      
      眼尖的读者可能注意会到，我们联合使用了`catch`操作符和`handleError`方法。
      但还没有讨论过它的实际工作方式。
  
      We use the Observable `catch` operator on the service level.
      It takes an error handling function with an error object as the argument.
  