Sky: Convert TransferMode to an enum.

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
index 88e9361b26a0e0007c5fb22a999c7ea12c68ff8b..e0b3ba062551d776e578ee9b1b65286ab2a9c3b0 100644
--- a/sky/engine/core/painting/TransferMode.dart
+++ b/sky/engine/core/painting/TransferMode.dart
@@ -4,48 +4,40 @@
 
 part of dart.sky;
 
-/// Color transfer mode.
-class TransferMode {
-  final int _value;
+/// List of predefined color transfer modes. This list comes from Skia's
+/// SkXfermode.h and the values (order) should be kept in sync.
+enum TransferMode {
+  clearMode,
+  srcMode,
+  dstMode,
+  srcOverMode,
+  dstOverMode,
+  srcInMode,
+  dstInMode,
+  srcOutMode,
+  dstOutMode,
+  srcATopMode,
+  dstATopMode,
+  xorMode,
+  plusMode,
+  modulateMode,
 
-  const TransferMode(this._value);
+  // Following blend modes are defined in the CSS Compositing standard.
+  screenMode,  /// The last coeff mode.
 
-  /// 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);
+  overlayMode,
+  darkenMode,
+  lightenMode,
+  colorDodgeMode,
+  colorBurnMode,
+  hardLightMode,
+  softLightMode,
+  differenceMode,
+  exclusionMode,
+  multiplyMode,  /// The last separable mode.
 
-  // 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);
+  hueMode,
+  saturationMode,
+  colorMode,
+  luminosityMode,
 }