[Effen] Add AnimatedComponent base class

sky/framework/components/fixed_height_scrollable.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/scroll_behavior.dart';
 import '../fn.dart';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'scrollable.dart';
 
@@ skipping 12 matching lines @@
     position:relative;
     will-change: transform;'''
   );
 
   double _height = 0.0;
   double _itemHeight;
 
   FixedHeightScrollable({
     Object key,
     ScrollBehavior scrollBehavior
-  }) : super(key: key, scrollBehavior: scrollBehavior);
+  }) : super(key: key, scrollBehavior: scrollBehavior) {
+    onDidMount(_measureHeights);
+  }
 
-  void didMount() {
-    super.didMount();
+  void _measureHeights() {
     var root = getRoot();
     var item = root.firstChild.firstChild;
     sky.ClientRect scrollRect = root.getBoundingClientRect();
     sky.ClientRect itemRect = item.getBoundingClientRect();
     assert(scrollRect.height > 0);
     assert(itemRect.height > 0);
 
     setState(() {
       _height = scrollRect.height;
       _itemHeight = itemRect.height;
@@ skipping 31 matching lines @@
       children: [
         new Container(
           style: _scrollAreaStyle,
           inlineStyle: transformStyle,
           children: buildItems(itemNumber, drawCount)
         )
       ]
     );
   }
 }