Change the way we provide custom dart code for IDL bindings.

sky/engine/core/painting/Gradient.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.
 
-// Extends the generated _Gradient interface via the CustomDart attribute.
+part of dart.sky;
 
+// Extends the generated _Gradient interface via the PrivateDart attribute.
 class Gradient extends _Gradient {
   // TODO(mpcomplete): Support other gradient types.
   // TODO(mpcomplete): Maybe pass a list of (color, colorStop) pairs instead?
   Gradient.Linear(List<Point> endPoints,
                   List<Color> colors,
                   List<double> colorStops)
       : super(0, endPoints, colors, _validate(colorStops, colors));
 
   // TODO(mpcomplete): Figure out a good way to validate arguments.
   static List<double> _validate(colorStops, colors) {
     if (colorStops != null && colors.length != colorStops.length) {
       throw new ArgumentError(
           "[colors] and [colorStops] parameters must be equal length.");
     }
     return colorStops;
   }
 }