anchor.diff:    
     bool collapsed = true;
     
  +  static const EventStreamProvider<CustomEvent> _polymerSelectEvent =
  +      const EventStreamProvider<CustomEvent>('polymer-select');
  +  
  +  Stream<CustomEvent> get onPolymerSelect =>
  +    PolymerUiSubmenuItem._polymerSelectEvent.forTarget(this);
  +  
     List<Element> get items {
       return (this.$['menu'] as PolymerUiMenu).items;
     }
  
positive.code: // http://www.polymer-project.org/. 
  library polymer_ui_elements.polymer_ui_submenu_item;
  
  import 'dart:html' show Element;
  import 'package:polymer/polymer.dart' show CustomTag, PolymerElement, published;
  import 'package:logging/logging.dart' show Logger;
  import 'package:polymer_ui_elements/polymer_ui_menu_item/polymer_ui_menu_item.dart' show PolymerUiMenuItem;
  import 'package:polymer_ui_elements/polymer_ui_menu/polymer_ui_menu.dart' show PolymerUiMenu;
  //import 'package:polymer_elements/polymer_selector/polymer_selector.dart' show PolymerSelector;
  
  /**
   * polymer-ui-submenu-item is a menu-item that can contains other menu-items.
  
negative.code:     if (this.hasItems() && this.active) {
        this.collapsed = !this.collapsed;
        this.unselectAllItems();
        dispatchEvent(new CustomEvent('polymer-select', detail: {'isSelected': true, 'item': this}));
      }
    }
    
  