Make it possible to detect PopupMenuItem presses

sky/sdk/lib/widgets/popup_menu_item.dart

Index: sky/sdk/lib/widgets/popup_menu_item.dart
diff --git a/sky/sdk/lib/widgets/popup_menu_item.dart b/sky/sdk/lib/widgets/popup_menu_item.dart
index 9888d4e296a3b35b06b0b0637b93de6be22a6827..d04e3266a405f2d0bf9ca5b9f9527532a858fb5b 100644
--- a/sky/sdk/lib/widgets/popup_menu_item.dart
+++ b/sky/sdk/lib/widgets/popup_menu_item.dart
@@ -12,16 +12,23 @@ const double kMenuItemHeight = 48.0;
 const double kBaselineOffsetFromBottom = 20.0;
 
 class PopupMenuItem extends Component {
-  PopupMenuItem({ String key, this.child, this.opacity}) : super(key: key);
+  PopupMenuItem({
+    String key,
+    this.onPressed,
+    this.child
+  }) : super(key: key);
 
   final Widget child;
-  final double opacity;
+  final Function onPressed;
 
   TextStyle get textStyle => Theme.of(this).text.subhead;
 
   Widget build() {
-    return new Opacity(
-      opacity: opacity,
+    return new Listener(
+      onGestureTap: (_) {
+        if (onPressed != null)
+          onPressed();
+      },
       child: new InkWell(
         child: new Container(
           height: kMenuItemHeight,