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

sky/engine/core/painting/TransferMode.dart

Index: sky/engine/core/painting/TransferMode.dart
diff --git a/sky/engine/core/painting/TransferMode.dart b/sky/engine/core/painting/TransferMode.dart
new file mode 100644
index 0000000000000000000000000000000000000000..88e9361b26a0e0007c5fb22a999c7ea12c68ff8b
--- /dev/null
+++ b/sky/engine/core/painting/TransferMode.dart
@@ -0,0 +1,51 @@
+// 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.
+
+part of dart.sky;
+
+/// Color transfer mode.
+class TransferMode {
+  final int _value;
+
+  const TransferMode(this._value);
+
+  /// List of predefined modes. This list comes from Skia's SkXfermode.h and
+  /// the values should be kept in sync.
+  static const TransferMode clearMode = const TransferMode(0);
+  static const TransferMode srcMode = const TransferMode(1);
+  static const TransferMode dstMode = const TransferMode(2);
+  static const TransferMode srcOverMode = const TransferMode(3);
+  static const TransferMode dstOverMode = const TransferMode(4);
+  static const TransferMode srcInMode = const TransferMode(5);
+  static const TransferMode dstInMode = const TransferMode(6);
+  static const TransferMode srcOutMode = const TransferMode(7);
+  static const TransferMode dstOutMode = const TransferMode(8);
+  static const TransferMode srcATopMode = const TransferMode(9);
+  static const TransferMode dstATopMode = const TransferMode(10);
+  static const TransferMode xorMode = const TransferMode(11);
+  static const TransferMode plusMode = const TransferMode(12);
+  static const TransferMode modulateMode = const TransferMode(13);
+
+  // Following blend modes are defined in the CSS Compositing standard:
+  // https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#blending
+  static const TransferMode screenMode = const TransferMode(14);
+  static const TransferMode lastCoeffMode = screenMode = const TransferMode(15);
+
+  static const TransferMode overlayMode = const TransferMode(16);
+  static const TransferMode darkenMode = const TransferMode(17);
+  static const TransferMode lightenMode = const TransferMode(18);
+  static const TransferMode colorDodgeMode = const TransferMode(19);
+  static const TransferMode colorBurnMode = const TransferMode(20);
+  static const TransferMode hardLightMode = const TransferMode(21);
+  static const TransferMode softLightMode = const TransferMode(22);
+  static const TransferMode differenceMode = const TransferMode(23);
+  static const TransferMode exclusionMode = const TransferMode(24);
+  static const TransferMode multiplyMode = const TransferMode(25);
+  static const TransferMode lastSeparableMode = multiplyMode = const TransferMode(26);
+
+  static const TransferMode hueMode = const TransferMode(27);
+  static const TransferMode saturationMode = const TransferMode(28);
+  static const TransferMode colorMode = const TransferMode(29);
+  static const TransferMode luminosityMode = const TransferMode(30);
+}