Begin work on the PopupMenu entrance animation

sky/framework/components/popup_menu_item.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 '../fn.dart';
 import 'material.dart';
 
 class PopupMenuItem extends Component {
   static final Style _style = new Style('''
     min-width: 112px;
     padding: 16px;''');
 
   List<Node> children;
+  AnimatedValueListener _opacity;
 
-  PopupMenuItem({ Object key, this.children }) : super(key: key);
+  PopupMenuItem({ Object key, this.children, AnimatedValue opacity}) : super(key: key) {
+    _opacity = new AnimatedValueListener(this, opacity);
+  }
+
+  void didUnmount() {
+    _opacity.stopListening();
+  }
 
   Node build() {
+    _opacity.ensureListening();
+
     return new Material(
       style: _style,
+      inlineStyle: _opacity.value == null ? null : 'opacity: ${_opacity.value}',
       children: children
     );
   }
 }