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

sky/engine/core/painting/DrawLooperLayerInfo.idl

 // 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.
 
 [
   Constructor(),
 ] interface DrawLooperLayerInfo {
   // Bits specifies which aspects of the layer's paint should replace the
   // corresponding aspects on the draw's paint.
   // ENTIRE_PAINT_BITS means use the layer's paint completely.
   // 0 means ignore the layer's paint... except for colorMode, which is
   // 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;
   const unsigned long MASK_FILTER_BIT  = 0x8;
   const unsigned long SHADER_BIT       = 0x10;
   const unsigned long COLOR_FILTER_BIT = 0x20;
   const unsigned long XFERMODE_BIT     = 0x40;
   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);
 };