Remove one more use of mirrors: Components now have to explicitly sync their fields.

sky/sdk/lib/framework/components2/popup_menu_item.dart

Index: sky/sdk/lib/framework/components2/popup_menu_item.dart
diff --git a/sky/sdk/lib/framework/components2/popup_menu_item.dart b/sky/sdk/lib/framework/components2/popup_menu_item.dart
index bf7d64d2757df67f590b1aa10672dee972efd4f6..f3fb84efc4f361d07a54a9d3c6641218012d892f 100644
--- a/sky/sdk/lib/framework/components2/popup_menu_item.dart
+++ b/sky/sdk/lib/framework/components2/popup_menu_item.dart
@@ -11,6 +11,12 @@ class PopupMenuItem extends Component {
   List<UINode> children;
   double opacity;
 
+  void syncFields(PopupMenuItem source) {
+    children = source.children;
+    opacity = source.opacity;
+    super.syncFields(source);
+  }
+
   UINode build() {
     return new Container(
       constraints: const BoxConstraints(minWidth: 112.0),