anchor.diff:  typedef Tween<T> TweenVisitor<T>(Tween<T> tween, T targetValue, TweenConstructor<T> constructor);
   
   /// A base class for widgets with implicit animations.
  +///
  +/// Subclasses must implement the [forEachTween] method to help
  +/// [AnimatedWidgetBaseState] iterate through the subclasses' widget's fields
  +/// and animate them.
   abstract class AnimatedWidgetBaseState<T extends ImplicitlyAnimatedWidget> extends State<T> with SingleTickerProviderStateMixin {
     AnimationController _controller;
   
  
positive.code: 
  /// A container that gradually changes its values over a period of time.
  ///
  /// This class is useful for generating simple implicit transitions between
  /// different parameters to [Container]. For more complex animations, you'll
  /// likely want to use a subclass of [Transition] or use an
  /// [AnimationController] yourself.
  ///
  /// Properties that are null are not animated.
  class AnimatedContainer extends ImplicitlyAnimatedWidget {
    /// Creates a container that animates its parameters implicitly.
    ///
  
negative.code: 
  /// An abstract widget for building widgets that gradually change their
  /// values over a period of time.
  ///
  /// Subclasses' States must provide a way to visit the subclass's relevant
  /// fields to animate. [ImplicitlyAnimatedWidget] will then automatically
  /// interpolate and animate those fields using the provided duration and
  /// curve when those fields change.
  abstract class ImplicitlyAnimatedWidget extends StatefulWidget {
    /// Initializes fields for subclasses.
    ///
  