anchor.diff:        :marked
           Retrieve a service from the current `TestBed` injector.
   
  +        从当前`TestBed`注入器获取一个服务。
  +
           The `inject` function is often adequate for this purpose. 
           But `inject` throws an error if it can't provide the service. 
           What if the service is optional?
   
  +        `inject`函数通常很适合这个任务。
  +        但是如果`inject`不能提供服务，它会抛出错误。
  +        如果服务是可选的呢？
  +
           The `TestBed.get` method takes an optional second parameter, 
           the object to return if Angular can't find the provider
           (`null` in this example):
  +
  +        `TestBed.get`方法接受一个可选的第二参数，它是在Angular找不到所需提供商时返回的对象。（在本例中为`null`）：
         +makeExample('testing/ts/app/bag/bag.spec.ts', 'testbed-get')(format=".")
         :marked
           Once called, the `TestBed` configuration is frozen for the duration of the current spec.
  +
  +        一旦调用，`TestBed`的配置就会在当前测试期间被冻结。
   
     tr
       td(style="vertical-align: top").
  
positive.code:         Replace metadata for the given `NgModule`. Recall that modules can import other modules.
          The `overrideModule` method can reach deeply into the current testing module to
          modify one of these inner modules.
    tr
      td(style="vertical-align: top") <code>overrideComponent</code>
      td
        :marked
          Replace metadata for the given component class which could be nested deeply 
          within an inner module.
    tr
      td(style="vertical-align: top") <code>overrideDirective</code>
      td
        :marked
          Replace metadata for the given directive class which could be nested deeply 
          within an inner module.
    tr
      td(style="vertical-align: top") <code>overridePipe</code>
      td
        :marked
          Replace metadata for the given pipe class which could be nested deeply 
          within an inner module.
    tr
      td(style="vertical-align: top").
        <a id="testbed-get"></a>
  
negative.code:       :marked
          Create an instance of a component of type `T` based on the current `TestBed` configuration.
          Once called, the `TestBed` configuration is frozen for the duration of the current spec.
  
          基于当前`TestBed`的配置创建一个类型为T的组件实例。
          一旦调用，`TestBed`的配置就会在当前测试期间被冻结。
    tr
      td(style="vertical-align: top") <code>overrideModule</code>
      td
  