Begin work on the PopupMenu entrance animation

sky/framework/components/popup_menu_item.dart

Index: sky/framework/components/popup_menu_item.dart
diff --git a/sky/framework/components/popup_menu_item.dart b/sky/framework/components/popup_menu_item.dart
index d404bf79e028cdbbcde545ba92b230040e493abd..8a5a7181bd23cb6662f37f79814d6b6e8bf360b0 100644
--- a/sky/framework/components/popup_menu_item.dart
+++ b/sky/framework/components/popup_menu_item.dart
@@ -2,6 +2,7 @@
 // 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';
 
@@ -11,12 +12,22 @@ class PopupMenuItem extends Component {
     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
     );
   }