Sky: Add a DartConverterEnum and use that for all our enum needs.

sky/engine/core/painting/Gradient.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.
 
 part of dart.sky;
 
 enum TileMode {
   clamp,
   repeated,
   mirror
 }
 
 // Extends the generated _Gradient interface via the PrivateDart attribute.
 class Gradient extends _Gradient {
   // TODO(mpcomplete): Maybe pass a list of (color, colorStop) pairs instead?
   Gradient.Linear(List<Point> endPoints,
                   List<Color> colors,
                   List<double> colorStops,
                   [TileMode tileMode = TileMode.clamp])
       : super() {
     if (endPoints == null || endPoints.length != 2)
       throw new ArgumentError("Expected exactly 2 [endPoints].");
     validateColorStops(colors, colorStops);
-    this._initLinear(endPoints, colors, colorStops, tileMode.index);
+    this._initLinear(endPoints, colors, colorStops, tileMode);
   }
 
   Gradient.Radial(Point center,
                   double radius,
                   List<Color> colors,
                   List<double> colorStops,
                   [TileMode tileMode = TileMode.clamp])
       : super() {
     validateColorStops(colors, colorStops);
-    this._initRadial(center, radius, colors, colorStops, tileMode.index);
+    this._initRadial(center, radius, colors, colorStops, tileMode);
   }
 
   void validateColorStops(List<Color> colors, List<double> colorStops) {
     if (colorStops != null && colors.length != colorStops.length) {
       throw new ArgumentError(
           "[colors] and [colorStops] parameters must be equal length.");
     }
   }
 }