anchor.diff:    void stop() {
       if (!isTicking)
         return;
  +
  +    _startTime = null;
   
       if (_animationId != null) {
         scheduler.cancelAnimationFrame(_animationId);
  
positive.code:     assert(simulation != null);
      assert(!_ticker.isTicking);
      _simulation = simulation;
      _startTime = null;
      _value = simulation.x(0.0);
      return _ticker.start();
    }
  
negative.code: 
    Completer _completer;
    int _animationId;
    Duration _startTime;
  
    /// Start calling onTick once per animation frame
    ///
    /// The returned future resolves once the ticker stops ticking.
    Future start() {
      assert(!isTicking);
      assert(_startTime == null);
      _completer = new Completer();
      _scheduleTick();
      return _completer.future;
  