anchor.diff:  /// A widget that can be copied by a [Mimic].
   ///
   /// This widget's State, [MimicableState], contains an API for initiating the
  -/// mimic operation.
  +/// mimic operation. Typically you would obtain this state by assigning a
  +/// [GlobalKey] to the [Mimicable] and then using [GlobalKey.currentState].
   class Mimicable extends StatefulWidget {
     /// Creates a widget that can be copies by a [Mimic].
     Mimicable({ Key key, this.child }) : super(key: key);
  
positive.code: 
    /// Start the mimicking process.
    ///
    /// The child of this object will no longer be built at this
    /// location in the tree. Instead, this widget will build a
    /// transparent placeholder with the same dimensions as the widget
    /// had when the mimicking process started.
    ///
    /// If you use startMimic(), it is your responsibility to do
    /// something with the returned [MimicableHandle]; typically,
    /// passing it to a [Mimic] widget. To mimic the child in the
    /// [Overlay], consider using [liftToOverlay()] instead.
    MimicableHandle startMimic() {
      assert(() {
        if (_placeholderSize != null) {
  
negative.code:     return new MimicableHandle._(this);
    }
  
    /// Start the mimicking process and mimic this object in the enclosing
    /// [Overlay].
    ///
    /// The child of this object will no longer be built at this location in the
    /// tree. Instead, (1) this widget will build a transparent placeholder with
    /// the same dimensions as the widget had when the mimicking process started
    /// and (2) the child will be placed in the enclosing overlay.
    ///
    /// To end the mimic process, call [MimicOverlayEntry.dispose] on the returned
    /// object.
    MimicOverlayEntry liftToOverlay() {
      OverlayState overlay = Overlay.of(context, debugRequiredFor: config);
      return new MimicOverlayEntry._(startMimic(), overlay);
  