anchor.diff:  
   /// The currently shown placeholder. 
   _Placeholder _currentPlaceholder;
  +
  +/// Flag indicating that draggable was dropped on a valid sortable target.
  +bool _dropped = false;
   
   /**
    * Manages a group of sortables and their options and event listeners.
  
positive.code:     ..install(placeholderElement)
      ..onDrop.listen((DropzoneEvent event) {
        _logger.finest('drop on placeholder');
        hidePlaceholder(dropped: true);
      });
    }
    
  
negative.code:   }
    
    void _listenToDragEnter() {
      _dragEnterSub = onDragEnter.listen((DropzoneEvent event) {
        _logger.finest('dragEnter');
          
        // Test if there already is a placeholder.
        if (_currentPlaceholder == null) {
          _currentPlaceholder = new _Placeholder(currentDraggable, 
              currentDraggableGroup);
        }
        
        // The first time this group is enterered during a drag, drop and 
        // dragEnd listeners are installed.
        if (_sortableSubs.isEmpty) {
          _listenToDrop();
          _listenToDragEnd(currentDraggableGroup);
        }
          
        // Show the placeholder for the entered dropzone.
  