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

sky/engine/core/painting/Gradient.idl

 // 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.
 
 [
     ImplementedAs=CanvasGradient,
     PrivateDart,
     Constructor()
 ] interface Gradient : Shader {
   void initLinear(Point[] endPoints, Color[] colors, float[] colorStops,
-                  /*TileMode*/unsigned long tileMode);
+                  TileMode tileMode);
   void initRadial(Point center, double radius, Color[] colors,
-                  float[] colorStops, /*TileMode*/unsigned long tileMode);
+                  float[] colorStops, TileMode tileMode);
 };