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

sky/engine/core/painting/TransferMode.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.
+
+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);

[eseidel, 2015/06/02 00:22:17]

Oh man.  We are gonna want some autogeneration or DSL-style assistance for these
blocks in the future.  Dart doesn't have a more concise way to write enums?

[abarth-chromium, 2015/06/02 00:24:40]

Dart has support for enums.  Maybe we just need a good way to access them via
the DarVM API?

[Matt Perry, 2015/06/02 18:28:54]

I could use enums for this. They have 2 disadvantages:

1. You can't specify the enum values. They always start from 0 and increment by
1.
2. I can't figure out how to create an enum in C++ given an integer.

Neither are relevant here. #1 is fine because these values happen to follow that
pattern anyway. #2 is fine because we don't return a TransferMode from C++ to
Dart anywhere.

I can change to enums if you think it's better. I went with this way because it
seemed more flexible.

+  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);
+}