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

sky/engine/core/painting/Canvas.cpp

Index: sky/engine/core/painting/Canvas.cpp
diff --git a/sky/engine/core/painting/Canvas.cpp b/sky/engine/core/painting/Canvas.cpp
index 0854cdba98501275b815342eb0268a8fb69f8890..f169066c1cb9febda8dda9c5e32aa81c63e590fe 100644
--- a/sky/engine/core/painting/Canvas.cpp
+++ b/sky/engine/core/painting/Canvas.cpp
@@ -120,7 +120,7 @@ void Canvas::drawPicture(Picture* picture)
     m_canvas->drawPicture(picture->displayList()->picture());
 }
 
-void Canvas::drawPaint(Paint* paint)
+void Canvas::drawPaint(const Paint* paint)
 {
     if (!m_canvas)
         return;
@@ -147,7 +147,7 @@ void Canvas::drawOval(const Vector<float>& rect, const Paint* paint)
     m_canvas->drawOval(toSkRect(rect), paint->paint());
 }
 
-void Canvas::drawCircle(float x, float y, float radius, Paint* paint)
+void Canvas::drawCircle(float x, float y, float radius, const Paint* paint)
 {
     if (!m_canvas)
         return;