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

sky/engine/core/painting/Paint.cpp

Index: sky/engine/core/painting/Paint.cpp
diff --git a/sky/engine/core/painting/Paint.cpp b/sky/engine/core/painting/Paint.cpp
index 549cd72dbd503a4a0f1b9cfb27ff790dd5b3641e..05164a09d8d555d5f1105ffce9271d9e12e02230 100644
--- a/sky/engine/core/painting/Paint.cpp
+++ b/sky/engine/core/painting/Paint.cpp
@@ -7,6 +7,7 @@
 
 #include "sky/engine/core/painting/ColorFilter.h"
 #include "sky/engine/core/painting/DrawLooper.h"
+#include "sky/engine/core/painting/MaskFilter.h"
 
 namespace blink {
 
@@ -30,4 +31,10 @@ void Paint::setColorFilter(ColorFilter* filter)
     m_paint.setColorFilter(filter->filter());
 }
 
+void Paint::setMaskFilter(MaskFilter* filter)
+{
+    ASSERT(filter);
+    m_paint.setMaskFilter(filter->filter());
+}
+
 } // namespace blink