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

sky/engine/core/painting/Gradient.idl

Index: sky/engine/core/painting/Gradient.idl
diff --git a/sky/engine/core/painting/Gradient.idl b/sky/engine/core/painting/Gradient.idl
index 343d6e18987011b1a04619acfdee82e5c68d9638..532e3a04e99f2a46bc6f0db8118d58284906cb21 100644
--- a/sky/engine/core/painting/Gradient.idl
+++ b/sky/engine/core/painting/Gradient.idl
@@ -8,7 +8,7 @@
     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);
 };