anchor.diff:    Here is the `TwainComponent`.
   
     本例的`About`视图显示马克吐温的名言。
  -  `TwainComponent`组件处理显示、委派`TwainService`向服务器发起请求。
  +  `TwainComponent`组件处理视图，并委派`TwainService`向服务器发起请求。
     两者都在`app/shared`目录里，因为作者计划将来在其它页面也显示马克吐温的名言。
     下面是`TwainComponent`：
   
  
positive.code:   This test has no direct access to the promise returned by the call to `testService.getQuote` 
    which is private and inaccessible inside `TwainComponent`.
  
    调用`testService.getQuote`返回的承诺是是私有的，不能被`TwainComponent`访问，这个测试程序也对其无访问权。
  
    Fortunately, the `getQuote` promise is accessible to the _async test zone_ 
    which intercepts all promises issued within the _async_ method call.
  
negative.code:   The `ComponentFixture.whenStable` method returns its own promise which resolves when the `getQuote` promise completes.
    In fact, the _whenStable_ promise resolves when _all pending asynchronous activities within this test_ complete ... the definition of "stable".
  
    `ComponentFixture.whenStable`方法返回它自己的承诺，它在`getQuote`承诺完成时被解析。实际上，“stable”的意思是当**所有待处理异步行为**完成时的状态，在“stable”后**whenStable**承诺被解析。
  
    Then the test resumes and kicks off another round of change detection (`fixture.detectChanges`)
    which tells Angular to update the DOM with the quote.
  