anchor.diff:  
   /// Animates its own size and clips and aligns the child.
   class SizeTransition extends AnimatedWidget {
  +  /// Creates a size transition.
  +  ///
  +  /// The [sizeFactor] argument is required. The [axis] argument defaults to
  +  /// [Axis.vertical]. The [axisAlignment] defaults to 0.5, which centers the
  +  /// child along the main axis during the transition.
     SizeTransition({
       Key key,
       this.axis: Axis.vertical,
  
positive.code: 
    @override
    Widget build(BuildContext context) {
      double scaleValue = scale.value;
      Matrix4 transform = new Matrix4.identity()
        ..scale(scaleValue, scaleValue);
      return new Transform(
        transform: transform,
        alignment: alignment,
  
negative.code: /// For more complex case involving additional state, consider using
  /// [AnimatedBuilder].
  abstract class AnimatedWidget extends StatefulWidget {
    /// The [animation] argument is required.
    AnimatedWidget({
      Key key,
      this.animation
  