Move AnimatedComponent away from mirrors and towards a callback-based setter.

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 2f9058f7ed5c2b669da51031bfd31be4f5079bd8..ef0ff3e53e264fabbd9061c0ec64a69119e9c4dc 100644
--- a/sky/sdk/lib/framework/components2/popup_menu.dart
+++ b/sky/sdk/lib/framework/components2/popup_menu.dart
@@ -61,7 +61,9 @@ class PopupMenu extends AnimatedComponent {
 
   PopupMenu({ Object key, this.controller, this.items, this.level })
       : super(key: key) {
-    animateField(controller.position, #_position);
+    animate(controller.position, (double value) {
+      _position = value;
+    });
     // onDidMount(_measureSize);
   }