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

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import '../animation/animated_value.dart';
 import '../fn2.dart';
 import '../theme2/colors.dart';
 import '../theme2/shadows.dart';
 import 'animated_component.dart';
 import 'dart:async';
@@ skipping 43 matching lines @@
   List<List<UINode>> items;
   int level;
   PopupMenuController controller;
 
   double _position;
   // int _width;
   // int _height;
 
   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);
   }
 
   double _opacityFor(int i) {
     if (_position == null || _position == 1.0)
       return null;
     double unit = 1.0 / items.length;
     double duration = 1.5 * unit;
     double start = i * unit;
     return math.max(0.0, math.min(1.0, (_position - start) / duration));
@@ skipping 30 matching lines @@
         padding: const EdgeDims.all(8.0),
         decoration: new BoxDecoration(
           backgroundColor: Grey[50],
           borderRadius: 2.0,
           boxShadow: Shadow[level]),
         child: new BlockContainer(children: children)
       )
     );
   }
 }