Add TransferMode to dart:sky, and use it instead of passing ints around.

sky/examples/raw/shadow.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:sky';
 
 void beginFrame(double timeStamp) {
   var size = 100.0;
   PictureRecorder canvas = new PictureRecorder(view.width, view.height);
   canvas.translate(size + 10.0, size + 10.0);
 
   Paint paint = new Paint();
   paint.setARGB(255, 0, 255, 0);
   var builder = new LayerDrawLooperBuilder()
     // Shadow layer.
     ..addLayerOnTop(
-        new DrawLooperLayerInfo()..setOffset(5.0, 5.0)..setColorMode(5),
+        new DrawLooperLayerInfo()
+          ..setOffset(const Point(5.0, 5.0))
+          ..setColorMode(TransferMode.srcInMode),
         (Paint layerPaint) {
       layerPaint.setARGB(128, 55, 55, 55);
       // TODO(mpcomplete): add blur filter
     })
     // Main layer.
     ..addLayerOnTop(new DrawLooperLayerInfo(), (Paint) {});
   paint.setDrawLooper(builder.build());
 
   canvas.drawPaint(new Paint()..setARGB(255, 255, 255, 255));
   canvas.drawRect(new Rect.fromLTRB(-size, -size, size, size), paint);
   view.picture = canvas.endRecording();
 }
 
 void main() {
   view.setBeginFrameCallback(beginFrame);
   view.scheduleFrame();
 }