anchor.diff:    final GlobalKey _subtreeKey = new GlobalKey();
     final PageStorageBucket _storageBucket = new PageStorageBucket();
   
  +  // one of the builders
     Widget _buildModalBarrier(BuildContext context) {
       Widget barrier;
       if (barrierColor != null) {
  
positive.code:     );
    }
  
    Widget _buildModalScope(BuildContext context) {
      return new _ModalScope(
        key: _scopeKey,
  
negative.code:       key: new GlobalObjectKey(config.route),
        child: new RepaintBoundary(child: contents)
      );
      return contents;
    }
  }
  
  /// A route that blocks interaction with previous routes.
  ///
  /// ModalRoutes cover the entire [Navigator]. They are not necessarily [opaque],
  /// however; for example, a pop-up menu uses a ModalRoute but only shows the menu
  /// in a small box overlapping the previous route.
  abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T> {
    ModalRoute({
      Completer<T> completer,
  