add initState, rename animated_container

sky/sdk/lib/widgets/flat_button.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 '../theme/colors.dart';
 import 'basic.dart';
 import 'material_button.dart';
 import 'theme.dart';
 
 class FlatButton extends MaterialButton {
@@ skipping 11 matching lines @@
     if (!enabled || !highlight)
       return null;
     switch (Theme.of(this).brightness) {
       case ThemeBrightness.light:
         return Grey[400];
       case ThemeBrightness.dark:
         return Grey[200];
     }
   }
 
-  int get level => null;
+  int get level => 0;
 }