Don't hardcode the list of events types in fn

sky/examples/fn/widgets/icon.dart

 part of widgets;
 
 const String kAssetBase = '/sky/assets/material-design-icons';
 
 class Icon extends Component {
 
   Style style;
   int size;
   String type;
-  sky.EventListener onClick;
 
   Icon({
     String key,
     this.style,
     this.size,
-    this.type: '',
-    this.onClick
+    this.type: ''
   }) : super(key: key);
 
   Node render() {
     String category = '';
     String subtype = '';
     List<String> parts = type.split('/');
     if (parts.length == 2) {
       category = parts[0];
       subtype = parts[1];
     }
 
     return new Image(
       style: style,
-      onClick: onClick,
       width: size,
       height: size,
       src: '${kAssetBase}/${category}/2x_web/ic_${subtype}_${size}dp.png'
     );
   }
 }