anchor.diff:  
   /// Animates the opacity of a widget.
   class FadeTransition extends AnimatedWidget {
  +  /// Creates an opacity transition.
  +  ///
  +  /// The [opacity] argument is required.
     FadeTransition({
       Key key,
       Animation<double> opacity,
  
positive.code: /// For simple cases without additional state, consider using
  /// [AnimatedWidget].
  class AnimatedBuilder extends AnimatedWidget {
    AnimatedBuilder({
      Key key,
      Animation<Object> animation,
  
negative.code: ///
  /// Only works if it's the child of a [Stack].
  class PositionedTransition extends AnimatedWidget {
    /// Creates a transition for [Positioned].
    ///
    /// The [rect] argument is required.
    PositionedTransition({
      Key key,
      Animation<RelativeRect> rect,
  