Revert "Add a Color class to dart:sky."

sky/engine/core/painting/Paint.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 Paint {
   attribute boolean isAntiAlias;
-  attribute Color color;
+  attribute unsigned long color;
 
-  // TODO(mpcomplete): remove this in favor of assigning to |color|.
   void setARGB(unsigned long a, unsigned long r, unsigned long g, unsigned long b);
   void setDrawLooper(DrawLooper looper);
   void setColorFilter(ColorFilter filter);
 };