Make the stocks popup menu fade in

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 96 matching lines @@
   // }
 
   UINode build() {
     int i = 0;
     List<UINode> children = new List.from(items.map((List<UINode> item) {
       double opacity = _opacityFor(i);
       return new PopupMenuItem(key: i++, children: item, opacity: opacity);
     }));
 
     // inlineStyle: _inlineStyle(),
-    return new ShrinkWrapWidth(
-      child: new Container(
-        padding: const EdgeDims.all(8.0),
-        decoration: new BoxDecoration(
-          backgroundColor: Grey[50],
-          borderRadius: 2.0,
-          boxShadow: Shadow[level]),
-        child: new Block(children)
+    return new Opacity(
+      opacity: math.min(1.0, _position * 3.0),
+      child: new ShrinkWrapWidth(
+        child: new Container(
+          padding: const EdgeDims.all(8.0),
+          decoration: new BoxDecoration(
+            backgroundColor: Grey[50],
+            borderRadius: 2.0,
+            boxShadow: Shadow[level]),
+          child: new Block(children)
+        )
       )
     );
   }
 
 }