anchor.diff:  
   /// Animates the position of a widget relative to its normal position.
   class SlideTransition extends AnimatedWidget {
  +  /// Creates a fractional translation transition.
  +  ///
  +  /// The [position] argument is required.
     SlideTransition({
       Key key,
       Animation<FractionalOffset> position,
       this.transformHitTests: true,
       this.child
  -  }) : position = position, super(key: key, animation: position) {
  -    assert(position != null);
  -  }
  +  }) : position = position, super(key: key, animation: position);
   
     /// The animation that controls the position of the child.
     ///
  
positive.code: ///
  /// Only works if it's the child of a [Stack].
  class PositionedTransition extends AnimatedWidget {
    PositionedTransition({
      Key key,
      Animation<RelativeRect> rect,
  
negative.code: 
    @override
    Widget build(BuildContext context) {
      final double scaleValue = scale.value;
      Matrix4 transform = new Matrix4.identity()
        ..scale(scaleValue, scaleValue, 1.0);
      return new Transform(
        transform: transform,
        alignment: alignment,
  