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:     _sortableSubs.add(onDrop.listen((DropzoneEvent event) {
        _logger.finest('drop');
        if (_currentPlaceholder != null) {
          // Just set dropped to true and let dragEnd handle it.
          _dropped = true;
        }
      }));
    }
    
    void _listenToDragEnd(DraggableGroup group) {
      // Clean up on dragEnd.
      _sortableSubs.add(group.onDragEnd.listen((_) {
        _logger.finest('dragEnd');
        
        // Cancel subscriptions.
        _sortableSubs.forEach((StreamSubscription s) => s.cancel());
        _sortableSubs.clear();
        
        // Hide placeholder if one is currently shown.
        if (_currentPlaceholder != null) {
          _currentPlaceholder.hidePlaceholder(dropped: _dropped);
        }
        
        // Reset dropped.
        _dropped = false;
      }));
    }
    
  