Sky: Allow clients to specify tile mode for gradients (repeating or mirror).

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 84e1f822451a76b2de0876d0905ba0797f5a0c53..343d6e18987011b1a04619acfdee82e5c68d9638 100644
--- a/sky/engine/core/painting/Gradient.idl
+++ b/sky/engine/core/painting/Gradient.idl
@@ -7,7 +7,8 @@
     PrivateDart,
     Constructor()
 ] interface Gradient : Shader {
-  void initLinear(Point[] endPoints, Color[] colors, float[] colorStops);
+  void initLinear(Point[] endPoints, Color[] colors, float[] colorStops,
+                  /*TileMode*/unsigned long tileMode);
   void initRadial(Point center, double radius, Color[] colors,
-                  float[] colorStops);
+                  float[] colorStops, /*TileMode*/unsigned long tileMode);
 };