anchor.diff:    TextStyle lerp(double t) => TextStyle.lerp(begin, end, t);
   }
   
  -/// 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.
  +/// An abstract class for building widgets that animate changes to their
  +/// properties.
  +///
  +/// Widgets of this type will not animate when they are first added to the
  +/// widget tree. Rather, when they are rebuilt with different values, they will
  +/// respond to those _changes_ by animating the changes over a specified
  +/// [duration].
  +///
  +/// Which properties are animated is left up to the subclass. Subclasses' States
  +/// must extend [ImplicitlyAnimatedWidgetState] and provide a way to visit the
  +/// relevant fields to animate.
   abstract class ImplicitlyAnimatedWidget extends StatefulWidget {
     /// Initializes fields for subclasses.
     ///
  
positive.code: ///
  /// This is the type of one of the arguments of [TweenVisitor], the signature
  /// used by [AnimatedWidgetBaseState.forEachTween].
  typedef TweenConstructor<T> = Tween<T> Function(T targetValue);
  
  /// Signature for callbacks passed to [AnimatedWidgetBaseState.forEachTween].
  typedef TweenVisitor<T> = Tween<T> Function(Tween<T> tween, T targetValue, TweenConstructor<T> constructor);
  
  /// A base class for widgets with implicit animations.
  ///
  /// [ImplicitlyAnimatedWidgetState] requires that subclasses respond to the
  /// animation, themselves. If you would like `setState()` to be called
  /// automatically as the animation changes, use [AnimatedWidgetBaseState].
  ///
  /// Subclasses must implement the [forEachTween] method to allow
  /// [ImplicitlyAnimatedWidgetState] to iterate through the subclasses' widget's
  /// fields and animate them.
  abstract class ImplicitlyAnimatedWidgetState<T extends ImplicitlyAnimatedWidget> extends State<T> with SingleTickerProviderStateMixin<T> {
    /// The animation controller driving this widget's implicit animations.
    @protected
  
negative.code:   ///
    /// {@tool sample}
    ///
    /// This sample implements an implicitly animated widget's `State`.
    /// The widget animates between colors whenever `widget.targetColor`
    /// changes.
    ///
  