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

sky/examples/raw/painting.sky

Index: sky/examples/raw/painting.sky
diff --git a/sky/examples/raw/painting.sky b/sky/examples/raw/painting.sky
index d8fe8f5eb5df7f8012704264f5cc7795db66eef0..70c519febe8098a194c79bd2a7c6ebe2373238dd 100644
--- a/sky/examples/raw/painting.sky
+++ b/sky/examples/raw/painting.sky
@@ -52,6 +52,8 @@ void main() {
           layerPaint.setColorFilter(
               new ColorFilter(const Color.fromARGB(128, 0, 0, 255),
                               TransferMode.srcInMode));
+          layerPaint.setMaskFilter(
+              new MaskFilter.Blur(BlurStyle.normal, 3.0, highQuality: true));
         })
         ..addLayerOnTop(
             new DrawLooperLayerInfo()