anchor.diff:  
   final _logger = new Logger("html5_dnd.sortable");
   
  +/// The currently shown placeholder. 
   _Placeholder _currentPlaceholder;
   
   /**
  
positive.code:   String placeholderClass = 'dnd-placeholder';
    
    /**
     * If true, forces the placeholder to have the computed size of the dragged
     * element. Default is true.
     * 
     * **Note:** Placeholders should have at least the size of the dragged 
  
negative.code:   /**
     * Installs sortable behaviour on [element] and registers it in this group.
     */
    @override
    void install(Element element) {
      _logger.finest('installing as sortable');
      // Sortable elements are at the same time draggables (superclass) and dropzones.
      
      // Install draggable behaviour.
  