Sky: Added radial gradients.

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 ba53d6c5525477e29a201fbcc919098f9f5a4393..84e1f822451a76b2de0876d0905ba0797f5a0c53 100644
--- a/sky/engine/core/painting/Gradient.idl
+++ b/sky/engine/core/painting/Gradient.idl
@@ -5,6 +5,9 @@
 [
     ImplementedAs=CanvasGradient,
     PrivateDart,
-    Constructor(unsigned long gradientType, Point[] endPoints, Color[] colors, float[] colorStops)
+    Constructor()
 ] interface Gradient : Shader {
+  void initLinear(Point[] endPoints, Color[] colors, float[] colorStops);
+  void initRadial(Point center, double radius, Color[] colors,
+                  float[] colorStops);
 };