Don't hardcode the list of events types in fn

sky/examples/fn/widgets/button.dart

 part of widgets;
 
 class Button extends ButtonBase {
 
   static Style _style = new Style('''
     transform: translateX(0);
     display: inline-flex;
     border-radius: 4px;
     justify-content: center;
     align-items: center;
     border: 1px solid blue;
     -webkit-user-select: none;
     margin: 5px;'''
   );
 
   static Style _highlightStyle = new Style('''
     transform: translateX(0);
     display: inline-flex;
     border-radius: 4px;
     justify-content: center;
     align-items: center;
     border: 1px solid blue;
     -webkit-user-select: none;
     margin: 5px;
     background-color: orange;'''
   );
 
   Node content;
-  sky.EventListener onClick;
 
-  Button({ Object key, this.content, this.onClick }) : super(key: key);
+  Button({ Object key, this.content }) : super(key: key);
 
   Node render() {
     return new Container(
       key: 'Button',
       style: _highlight ? _highlightStyle : _style,
-      onClick: onClick,
-      onPointerDown: _handlePointerDown,
-      onPointerUp: _handlePointerUp,
-      onPointerCancel: _handlePointerCancel,
       children: [super.render(), content]
     );
   }
 }