Style guide says enum values should be lowerCamelCase.

sky/sdk/lib/framework/components2/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 'dart:sky' as sky;
 import '../fn2.dart';
 import '../rendering/box.dart';
 import '../rendering/flex.dart';
 import 'button_base.dart';
 import 'icon.dart';
@@ skipping 16 matching lines @@
       new Container(
         child: new InkWell(
           children: [
             new Padding(
               child: new Icon(type: "${icon}_grey600", size: 24),
               padding: const EdgeDims.symmetric(horizontal: 16.0)
             ),
             new FlexExpandingChild(
               new Padding(
                 child: new FlexContainer(
-                  direction: FlexDirection.Horizontal,
+                  direction: FlexDirection.horizontal,
                   children: children
                 ),
                 padding: const EdgeDims.symmetric(horizontal: 16.0)
               ),
               1
             )
           ]
         ),
         desiredSize: const sky.Size.fromHeight(48.0),
         decoration: highlight ? highlightDecoration : null
       ),
       onGestureTap: onGestureTap
     );
   }
 }