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