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

sky/engine/core/painting/Canvas.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKY_ENGINE_CORE_PAINTING_CANVAS_H_
 #define SKY_ENGINE_CORE_PAINTING_CANVAS_H_
 
 #include "sky/engine/core/painting/Paint.h"
 #include "sky/engine/core/painting/Picture.h"
 #include "sky/engine/platform/graphics/DisplayList.h"
@@ skipping 21 matching lines @@
 
     void translate(float dx, float dy);
     void scale(float sx, float sy);
     void rotateDegrees(float degrees);
     void skew(float sx, float sy);
     void concat(const Vector<float>& matrix);
 
     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; }
 
 protected:
     PassRefPtr<DisplayList> finishRecording();
 
     bool isRecording() const { return m_canvas; }
 
 private:
     FloatSize m_size;
     RefPtr<DisplayList> m_displayList;
     SkCanvas* m_canvas;
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_PAINTING_CANVAS_H_