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

sky/engine/core/painting/DrawLooperLayerInfo.idl

Index: sky/engine/core/painting/DrawLooperLayerInfo.idl
diff --git a/sky/engine/core/painting/DrawLooperLayerInfo.idl b/sky/engine/core/painting/DrawLooperLayerInfo.idl
index f46c2bf450d6db59790d1d08de1bcef64a8a1980..8cb47152ab9ce9a699b12caabc5561aac583399f 100644
--- a/sky/engine/core/painting/DrawLooperLayerInfo.idl
+++ b/sky/engine/core/painting/DrawLooperLayerInfo.idl
@@ -12,6 +12,7 @@
   // always applied.
   // TODO(mpcomplete): maybe these should each be functions (e.g. useStyle()).
   // TODO(mpcomplete): the IDL compiler doesn't use these for anything?
+  // TODO(mpcomplete): dart style says to name these like 'styleBit'.
   const unsigned long STYLE_BIT        = 0x1;
   const unsigned long TEXT_SKEWX_BIT   = 0x2;
   const unsigned long PATH_EFFECT_BIT  = 0x4;
@@ -22,9 +23,7 @@
   const unsigned long ENTIRE_PAINT_BITS = -1;
 
   void setPaintBits(unsigned long bits);
-  // TODO(mpcomplete): use SkXfermode::Mode.
-  void setColorMode(unsigned long mode);
-  // TODO(mpcomplete): use Point.
-  void setOffset(float x, float y);
+  void setColorMode(TransferMode mode);
+  void setOffset(Point offset);
   void setPostTranslate(boolean postTranslate);
 };