Add a MaskFilter interface to dart:sky to handle blur.

sky/engine/core/painting/Paint.h

Index: sky/engine/core/painting/Paint.h
diff --git a/sky/engine/core/painting/Paint.h b/sky/engine/core/painting/Paint.h
index 9826515a8cc9dc2ab345329e3a314351f5355be0..a8af3cac141b6192529dfbee68c4a700bb292546 100644
--- a/sky/engine/core/painting/Paint.h
+++ b/sky/engine/core/painting/Paint.h
@@ -15,6 +15,7 @@ namespace blink {
 
 class DrawLooper;
 class ColorFilter;
+class MaskFilter;
 
 class Paint : public RefCounted<Paint>, public DartWrappable {
     DEFINE_WRAPPERTYPEINFO();
@@ -38,6 +39,7 @@ public:
     }
     void setDrawLooper(DrawLooper* looper);
     void setColorFilter(ColorFilter* filter);
+    void setMaskFilter(MaskFilter* filter);
 
     const SkPaint& paint() const { return m_paint; }
     void setPaint(const SkPaint& paint) { m_paint = paint; }