anchor.diff:   * A simple stopwatch interface to measure elapsed time.
    */
   class Stopwatch {
  +  /**
  +   * Frequency of the elapsed counter in Hz.
  +   */
  +  final int frequency = _frequency();
  +
     // The _start and _stop fields capture the time when [start] and [stop]
     // are called respectively.
     // If _start is null, then the [Stopwatch] has not been started yet.
  
positive.code:    *
     *     Stopwatch stopwatch = new Stopwatch()..start();
     */
    Stopwatch() : _start = null, _stop = null {}
  
    /**
     * Starts the [Stopwatch].
  
negative.code:     return (elapsedTicks * 1000) ~/ frequency;
    }
  
  
    /**
     * Returns wether the [StopWatch] is currently running.
  