anchor.diff:  class PositionedTransition extends AnimatedWidget {
     /// Creates a transition for [Positioned].
     ///
  -  /// The [rect] argument is required.
  +  /// The [rect] argument must not be null.
     PositionedTransition({
       Key key,
  -    Animation<RelativeRect> rect,
  +    @required Animation<RelativeRect> rect,
       @required this.child,
     }) : super(key: key, listenable: rect);
   
  
positive.code: 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,
      Animation<double> sizeFactor,
      this.axisAlignment: 0.5,
      this.child,
    }) : super(key: key, listenable: sizeFactor) {
  
negative.code: class RotationTransition extends AnimatedWidget {
    /// Creates a rotation transition.
    ///
    /// The [turns] argument must not be null.
    RotationTransition({
      Key key,
      @required Animation<double> turns,
      this.child,
    }) : super(key: key, listenable: turns);
  
  