Sky: Add a CustomDart attribute to the IDL compiler, and use that with Canvas

sky/engine/core/painting/Canvas.h

Index: sky/engine/core/painting/Canvas.h
diff --git a/sky/engine/core/painting/Canvas.h b/sky/engine/core/painting/Canvas.h
index e22d04910c06c29d4134a8e876aaf5c308dfe2b3..ca8af6e29ec4e494316248455a84bf2f01ad81d6 100644
--- a/sky/engine/core/painting/Canvas.h
+++ b/sky/engine/core/painting/Canvas.h
@@ -39,11 +39,10 @@ public:
     void clipRect(const Vector<float>& rect);
 
     void drawPicture(Picture* picture);
-
-    void drawPaint(Paint* paint);
+    void drawPaint(const Paint* paint);
     void drawRect(const Vector<float>& rect, const Paint* paint);
     void drawOval(const Vector<float>& rect, const Paint* paint);
-    void drawCircle(float x, float y, float radius, Paint* paint);
+    void drawCircle(float x, float y, float radius, const Paint* paint);
 
     SkCanvas* skCanvas() { return m_canvas; }