anchor.diff:    
     PolymerSelection.created() : super.created();
     
  -  static const EventStreamProvider<CustomEvent> _polymerSelectEvent =
  -      const EventStreamProvider<CustomEvent>('polymer-select');
  -  
  -  Stream<CustomEvent> get onPolymerSelect =>
  -      PolymerSelection._polymerSelectEvent.forTarget(this);
   
     /**
      * Retrieves the selected item(s). If the multi property is true,
  
positive.code:           this._selection.removeAt(i);
          }
        }     
   
        dispatchEvent(new CustomEvent('polymer-select', 
            detail: {'item': item, 'isSelected':  isSelected}));
        //this.fire('polymerselect',detail: {'item': item, 'isSelected':  isSelected});
            
      }
    }
  
negative.code: 
  library polymer_elements.polymer_selection;
  
  import 'package:polymer/polymer.dart';
  
  @CustomTag('polymer-selection')
  