attempt at making menuitems react

sky/examples/fn/widgets/buttonbase.dart

 part of widgets;
 
-abstract class ButtonBase extends Component {
+abstract class ButtonBase extends MaterialComponent {
 
   bool _highlight = false;
 
   ButtonBase({ Object key }) : super(key: key);
 
   void _handlePointerDown(_) {
     setState(() {
       _highlight = true;
     });
   }
   void _handlePointerUp(_) {
     setState(() {
       _highlight = false;
     });
   }
   void _handlePointerCancel(_) {
     setState(() {
       _highlight = false;
     });
   }
 }