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

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

Index: sky/sdk/lib/framework/components2/popup_menu.dart
diff --git a/sky/sdk/lib/framework/components2/popup_menu.dart b/sky/sdk/lib/framework/components2/popup_menu.dart
index 760f0e2d4434093f2340b3aaa25f111a96fcf707..e11a528e2f1d50771657c7af6aefa15d57b3fa8c 100644
--- a/sky/sdk/lib/framework/components2/popup_menu.dart
+++ b/sky/sdk/lib/framework/components2/popup_menu.dart
@@ -60,9 +60,16 @@ class PopupMenu extends AnimatedComponent {
     // onDidMount(_measureSize);
   }
 
+  PopupMenuController controller;
   List<List<UINode>> items;
   int level;
-  PopupMenuController controller;
+
+  void syncFields(PopupMenu source) {
+    controller = source.controller;
+    items = source.items;
+    level = source.level;
+    super.syncFields(source);
+  }
 
   double _position;
   // int _width;