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:    *
     * @return the [TimeCounterHandle] that should be used to stop counting.
     */
    TimeCounter_TimeCounterHandle start() => new TimeCounter_TimeCounterHandle(this);
  }
  
  /**
  
negative.code:  */
  class TimeCounter_TimeCounterHandle {
    final TimeCounter _counter;
    TimeCounter _prev;
  
    TimeCounter_TimeCounterHandle(this._counter) {
      // if there is some counter running, pause it
      _prev = TimeCounter._current;
      if (_prev != null) {
        _prev._sw.stop();
      }
      TimeCounter._current = _counter;
      // start this counter
      _counter._sw.start();
    }
  
  