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

For this, I added back my previous CustomDart attribute for customizing the
generated Dart code of an interface.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1162843003

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()

examples/raw/shadow.dart

@@ -10,22 +10,25 @@ void beginFrame(double timeStamp) {
   canvas.translate(size + 10.0, size + 10.0);
 
   Paint paint = new Paint();
-  paint.setARGB(255, 0, 255, 0);
+  paint.color = const Color.fromARGB(255, 0, 255, 0);
   var builder = new LayerDrawLooperBuilder()
     // Shadow layer.
     ..addLayerOnTop(
         new DrawLooperLayerInfo()
+          ..setPaintBits(-1)
           ..setOffset(const Point(5.0, 5.0))
           ..setColorMode(TransferMode.srcInMode),
         (Paint layerPaint) {
-      layerPaint.setARGB(128, 55, 55, 55);
-      // TODO(mpcomplete): add blur filter
+      layerPaint.color = const Color.fromARGB(128, 55, 55, 55);
+      layerPaint.setMaskFilter(
+          new MaskFilter.Blur(BlurStyle.normal, 5.0, highQuality: true));
     })
     // Main layer.
     ..addLayerOnTop(new DrawLooperLayerInfo(), (Paint) {});
   paint.setDrawLooper(builder.build());
 
-  canvas.drawPaint(new Paint()..setARGB(255, 255, 255, 255));
+  canvas.drawPaint(
+      new Paint()..color = const Color.fromARGB(255, 255, 255, 255));
   canvas.drawRect(new Rect.fromLTRB(-size, -size, size, size), paint);
   view.picture = canvas.endRecording();
 }