anchor.diff:     *
      *     Stopwatch stopwatch = new Stopwatch()..start();
      */
  -  Stopwatch() : _start = null, _stop = null {}
  +  Stopwatch();
   
     /**
      * Starts the [Stopwatch].
  
positive.code:     return (elapsedTicks * 1000) ~/ frequency;
    }
  
    /**
     * Returns the frequency of the elapsed counter in Hz.
     */
    int get frequency => _frequency();
  
    /**
     * Returns wether the [StopWatch] is currently running.
  
negative.code:  * 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.
  