[Effen] Add AnimatedComponent base class

sky/framework/components/fixed_height_scrollable.dart

Index: sky/framework/components/fixed_height_scrollable.dart
diff --git a/sky/framework/components/fixed_height_scrollable.dart b/sky/framework/components/fixed_height_scrollable.dart
index 948a6dfc8813e0e6c139577fae59725b268102f1..500af2e27f6b73f9ce3bf73231eba97c69f55eeb 100644
--- a/sky/framework/components/fixed_height_scrollable.dart
+++ b/sky/framework/components/fixed_height_scrollable.dart
@@ -30,10 +30,11 @@ abstract class FixedHeightScrollable extends Scrollable {
   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();