Change how events are handled in Effen

sky/framework/components/button_base.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import '../fn.dart';
 
 abstract class ButtonBase extends Component {
   bool highlight = false;
 
-  ButtonBase({ Object key }) : super(key: key) {
-    events.listen('pointerdown', _handlePointerDown);
-    events.listen('pointerup', _handlePointerUp);
-    events.listen('pointercancel', _handlePointerCancel);
+  ButtonBase({ Object key }) : super(key: key);
+
+  Node buildContent();
+
+  Node build() {
+    return new EventTarget(
+      buildContent(),
+      onPointerDown: _handlePointerDown,
+      onPointerUp: _handlePointerUp,
+      onPointerCancel: _handlePointerCancel
+    );
   }
 
   void _handlePointerDown(_) {
     setState(() {
       highlight = true;
     });
   }
   void _handlePointerUp(_) {
     setState(() {
       highlight = false;
     });
   }
   void _handlePointerCancel(_) {
     setState(() {
       highlight = false;
     });
   }
 }