anchor.diff:    /// The current value of the animation.
     ///
     /// Setting this value stops the current animation.
  -  double get value => _value.clamp(lowerBound, upperBound);
  +  double get value => _value;
     double _value;
     void set value(double newValue) {
       assert(newValue != null);
  
positive.code:     assert(simulation != null);
      assert(!isAnimating);
      _simulation = simulation;
      _value = simulation.x(0.0);
      return _ticker.start();
    }
  
  
negative.code: 
    void _tick(Duration elapsed) {
      double elapsedInSeconds = elapsed.inMicroseconds.toDouble() / Duration.MICROSECONDS_PER_SECOND;
      _value = _simulation.x(elapsedInSeconds).clamp(lowerBound, upperBound);
      if (_simulation.isDone(elapsedInSeconds))
        stop();
      notifyListeners();
  