anchor.diff:     *
      * @return the [TimeCounterHandle] that should be used to stop counting.
      */
  -  TimeCounter_TimeCounterHandle start() => new TimeCounter_TimeCounterHandle(this);
  +  TimeCounter_TimeCounterHandle start() {
  +    return new TimeCounter_TimeCounterHandle(this);
  +  }
   }
   
   /**
  
positive.code:  * Helper for measuring how much time is spent doing some operation.
   */
  class TimeCounter {
    Stopwatch _sw = new Stopwatch();
  
    /**
     * @return the number of milliseconds spent between [start] and [stop].
  
negative.code:    */
    void stop() {
      _counter._sw.stop();
      // restore previous counter and resume it
      TimeCounter._current = _prev;
      if (_prev != null) {
        _prev._sw.start();
      }
    }
  }
  