Don't hardcode the list of events types in fn

sky/examples/fn/widgets/button.dart

Index: sky/examples/fn/widgets/button.dart
diff --git a/sky/examples/fn/widgets/button.dart b/sky/examples/fn/widgets/button.dart
index ac7f62d62067af015d356c474139abe36d274e07..c7f53cac5baad6d88e0d631d57b205d8c2894fff 100644
--- a/sky/examples/fn/widgets/button.dart
+++ b/sky/examples/fn/widgets/button.dart
@@ -26,18 +26,13 @@ class Button extends ButtonBase {
   );
 
   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]
     );
   }