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

sky/engine/core/painting/MaskFilter.idl

Index: sky/engine/core/painting/MaskFilter.idl
diff --git a/sky/engine/core/painting/ColorFilter.idl b/sky/engine/core/painting/MaskFilter.idl
similarity index 61%
copy from sky/engine/core/painting/ColorFilter.idl
copy to sky/engine/core/painting/MaskFilter.idl
index 23d49f1a53042ae6bae9318bebb0a004fa0d9e50..fe43b562d85426fa2a6919394637108824141255 100644
--- a/sky/engine/core/painting/ColorFilter.idl
+++ b/sky/engine/core/painting/MaskFilter.idl
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 [
-  Constructor(Color color, TransferMode transferMode),
-] interface ColorFilter {
+  CustomDart,
+  Constructor(unsigned long style, double sigma, unsigned long flags)
+] interface MaskFilter {
 };