Sky: Add a DrawLooper interface to the painting API to be used for shadows.

sky/examples/raw/painting.sky

 <sky>
 <style>
 div {
     height: 200px;
     background-color: lightblue;
 }
 </style>
 <div id="canvas" />
 <script>
 import 'dart:math' as math;
@@ skipping 22 matching lines @@
         0.5, 0.0, 0.0,
         0.0, 0.5, 0.0,
         0.0, 0.0, 1.0
     ];
     context.concat(scaleMatrix);
     paint.setARGB(128, 0, 255, 0);
     context.drawCircle(0.0, 0.0, radius, paint);
 
     context.restore();
 
+    var builder = new LayerDrawLooperBuilder()
+        ..addLayerOnTop(
+            new DrawLooperLayerInfo()..setOffset(150.0, 0.0)..setPaintBits(-1),
+            (Paint layerPaint) {
+          // TODO(mpcomplete): This won't do anything until we add support for
+          // setting the color filter.
+          layerPaint.setARGB(128, 0, 0, 255);
+        })
+        ..addLayerOnTop(
+            new DrawLooperLayerInfo()..setOffset(75.0, 75.0)..setPaintBits(0),
+            (Paint layerPaint) {
+          layerPaint.setARGB(128, 255, 0, 0);
+        });
+    paint.setDrawLooper(builder.build());
     context.drawCircle(0.0, 0.0, radius, paint);
 
     context.commit();
   });
 }
 </script>
 </sky>