Expose and use constants for DrawLooperLayerInfo.setPaintBits to dart.

sky/examples/raw/painting.sky

 <sky>
 <style>
 div {
     height: 200px;
     background-color: lightblue;
 }
 </style>
 <div id="canvas" />
 <script>
 import 'dart:math' as math;
@@ skipping 33 matching lines @@
     context.drawCircle(0.0, 0.0, radius, paint);
 
     context.restore();
 
     context.translate(0.0, 50.0);
     var builder = new LayerDrawLooperBuilder()
         ..addLayerOnTop(
             new DrawLooperLayerInfo()
               ..setOffset(const Point(150.0, 0.0))
               ..setColorMode(TransferMode.srcMode)
-              ..setPaintBits(-1),
+              ..setPaintBits(PaintBits.all),
             (Paint layerPaint) {
           layerPaint.color = const Color.fromARGB(128, 255, 255, 0);
           layerPaint.setColorFilter(
               new ColorFilter.mode(const Color.fromARGB(128, 0, 0, 255),
                                    TransferMode.srcInMode));
           layerPaint.setMaskFilter(
               new MaskFilter.blur(BlurStyle.normal, 3.0, highQuality: true));
         })
         ..addLayerOnTop(
             new DrawLooperLayerInfo()
               ..setOffset(const Point(75.0, 75.0))
               ..setColorMode(TransferMode.srcMode)
-              ..setPaintBits(-1),
+              ..setPaintBits(PaintBits.shader),
             (Paint layerPaint) {
           Gradient redYellow = new Gradient.radial(
               new Point(0.0, 0.0), radius/3.0,
               [const Color(0xFFFFFF00), const Color(0xFFFF0000)],
               null, TileMode.mirror);
           layerPaint.setShader(redYellow);
+          // Since we're don't set PaintBits.maskFilter, this has no effect.
+          layerPaint.setMaskFilter(
+              new MaskFilter.blur(BlurStyle.normal, 50.0, highQuality: true));
         })
         ..addLayerOnTop(
             new DrawLooperLayerInfo()..setOffset(const Point(225.0, 75.0)),
             (Paint layerPaint) {
           // Since this layer uses a DST color mode, this has no effect.
           layerPaint.color = const Color.fromARGB(128, 255, 0, 0);
         });
     paint.setDrawLooper(builder.build());
     context.drawCircle(0.0, 0.0, radius, paint);
 
     context.commit();
   });
 }
 </script>
 </sky>