anchor.diff:  class ScaleTransition extends AnimatedWidget {
     /// Creates a scale transition.
     ///
  -  /// The [scale] argument is required. The [alignment] argument defaults to
  -  /// [FractionalOffset.center].
  +  /// The [scale] argument must not be null. The [alignment] argument defaults
  +  /// to [FractionalOffset.center].
     ScaleTransition({
       Key key,
  -    Animation<double> scale,
  +    @required Animation<double> scale,
       this.alignment: FractionalOffset.center,
       this.child,
     }) : super(key: key, listenable: scale);
  
positive.code: class FadeTransition extends AnimatedWidget {
    /// Creates an opacity transition.
    ///
    /// The [opacity] argument is required.
    FadeTransition({
      Key key,
      Animation<double> opacity,
      this.child,
    }) : super(key: key, listenable: opacity);
  
  
negative.code: class PositionedTransition extends AnimatedWidget {
    /// Creates a transition for [Positioned].
    ///
    /// The [rect] argument must not be null.
    PositionedTransition({
      Key key,
      @required Animation<RelativeRect> rect,
      @required this.child,
    }) : super(key: key, listenable: rect);
  
  