anchor.diff:  
     void _handleSizeChanged(Size newSize) {
       _width = newSize.width;
  -    _transform.position.end = _activeCardDragEndPoint;
  +    _position.end = _activeCardDragEndPoint;
     }
   
     void _handlePointerDown(sky.PointerEvent event) {
  
positive.code: 
  import 'dart:sky' as sky;
  
  import 'package:vector_math/vector_math.dart';
  import 'package:sky/animation/animation_performance.dart';
  import 'package:sky/widgets/animation_builder.dart';
  import 'package:sky/widgets/animated_component.dart';
  import 'package:sky/widgets/basic.dart';
  import 'package:sky/widgets/widget.dart';
  
  const int _kCardDismissFadeoutMS = 300;
  const double _kMinFlingVelocity = 700.0;
  const double _kMinFlingVelocityDelta = 400.0;
  const double _kDismissCardThreshold = 0.6;
  
negative.code:   Widget child;
    DismissedCallback onDismissed;
  
    AnimatedType<Point> _position;
    AnimatedType<double> _opacity;
    AnimationPerformance _performance;
  
    double _width;
    double _dragX = 0.0;
    bool _dragUnderway = false;
  
    void initState() {
      _position = new AnimatedType<Point>(Point.origin);
      _opacity = new AnimatedType<double>(1.0, end: 0.0);
      _performance = new AnimationPerformance()
        ..duration = _kCardDismissFadeout
        ..variable = new AnimatedList([_position, _opacity])
        ..addListener(_handleAnimationProgressChanged);
      watch(_performance);
    }
  
  