Style guide says enum values should be lowerCamelCase.

sky/sdk/lib/framework/rendering/flex.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 'box.dart';
 import 'object.dart';
 
 class FlexBoxParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> {
   int flex;
   void merge(FlexBoxParentData other) {
     if (other.flex != null)
       flex = other.flex;
     super.merge(other);
   }
 }
 
-enum FlexDirection { Horizontal, Vertical }
+enum FlexDirection { horizontal, vertical }
 
 class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, FlexBoxParentData>,
                                         RenderBoxContainerDefaultsMixin<RenderBox, FlexBoxParentData> {
   // lays out RenderBox children using flexible layout
 
   RenderFlex({
-    FlexDirection direction: FlexDirection.Horizontal
+    FlexDirection direction: FlexDirection.horizontal
   }) : _direction = direction;
 
   FlexDirection _direction;
   FlexDirection get direction => _direction;
   void set direction (FlexDirection value) {
     if (_direction != value) {
       _direction = value;
       markNeedsLayout();
     }
   }
@@ skipping 13 matching lines @@
   int _getFlex(RenderBox child) {
     assert(child.parentData is FlexBoxParentData);
     return child.parentData.flex != null ? child.parentData.flex : 0;
   }
 
   void performLayout() {
     // Based on http://www.w3.org/TR/css-flexbox-1/ Section 9.7 Resolving Flexible Lengths
     // Steps 1-3. Determine used flex factor, size inflexible items, calculate free space
     int totalFlex = 0;
     assert(constraints != null);
-    double freeSpace = (_direction == FlexDirection.Horizontal) ? constraints.maxWidth : constraints.maxHeight;
+    double freeSpace = (_direction == FlexDirection.horizontal) ? constraints.maxWidth : constraints.maxHeight;
     RenderBox child = firstChild;
     while (child != null) {
       int flex = _getFlex(child);
       if (flex > 0) {
         totalFlex += child.parentData.flex;
       } else {
         BoxConstraints innerConstraints = new BoxConstraints(maxHeight: constraints.maxHeight,
                                                              maxWidth: constraints.maxWidth);
         child.layout(innerConstraints, parentUsesSize: true);
-        freeSpace -= (_direction == FlexDirection.Horizontal) ? child.size.width : child.size.height;
+        freeSpace -= (_direction == FlexDirection.horizontal) ? child.size.width : child.size.height;
       }
       child = child.parentData.nextSibling;
     }
 
     // Steps 4-5. Distribute remaining space to flexible children.
     double spacePerFlex = totalFlex > 0 ? (freeSpace / totalFlex) : 0.0;
     double usedSpace = 0.0;
     child = firstChild;
     while (child != null) {
       int flex = _getFlex(child);
       if (flex > 0) {
         double spaceForChild = spacePerFlex * flex;
         BoxConstraints innerConstraints;
         switch (_direction) {
-          case FlexDirection.Horizontal:
+          case FlexDirection.horizontal:
             innerConstraints = new BoxConstraints(maxHeight: constraints.maxHeight,
                                                   minWidth: spaceForChild,
                                                   maxWidth: spaceForChild);
             break;
-          case FlexDirection.Vertical:
+          case FlexDirection.vertical:
             innerConstraints = new BoxConstraints(minHeight: spaceForChild,
                                                   maxHeight: spaceForChild,
                                                   maxWidth: constraints.maxWidth);
             break;
         }
         child.layout(innerConstraints, parentUsesSize: true);
       }
 
       // For now, center the flex items in the cross direction
       switch (_direction) {
-        case FlexDirection.Horizontal:
+        case FlexDirection.horizontal:
           child.parentData.position = new sky.Point(usedSpace, size.height / 2.0 - child.size.height / 2.0);
           usedSpace += child.size.width;
           break;
-        case FlexDirection.Vertical:
+        case FlexDirection.vertical:
           child.parentData.position = new sky.Point(size.width / 2.0 - child.size.width / 2.0, usedSpace);
           usedSpace += child.size.height;
           break;
       }
       child = child.parentData.nextSibling;
     }
   }
 
   void hitTestChildren(HitTestResult result, { sky.Point position }) {
     defaultHitTestChildren(result, position: position);
   }
 
   void paint(RenderObjectDisplayList canvas) {
     defaultPaint(canvas);
   }
 }