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

sky/engine/core/painting/MaskFilter.dart

Index: sky/engine/core/painting/MaskFilter.dart
diff --git a/sky/engine/core/painting/MaskFilter.dart b/sky/engine/core/painting/MaskFilter.dart
index c7014509bac72f29d91c4b5570f41c2fc84f769b..657cb36efd304517c953a7141eb460ce19cd883f 100644
--- a/sky/engine/core/painting/MaskFilter.dart
+++ b/sky/engine/core/painting/MaskFilter.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Extends the generated _MaskFilter interface via the CustomDart attribute.
+part of dart.sky;
 
 /// Blur styles. These mirror SkBlurStyle and must be kept in sync.
 enum BlurStyle {
@@ -12,6 +12,7 @@ enum BlurStyle {
   inner,  /// Fuzzy inside, nothing outside.
 }
 
+// Extends the generated _MaskFilter interface via the PrivateDart attribute.
 class MaskFilter extends _MaskFilter {
   MaskFilter.Blur(BlurStyle style, double sigma,
                   {bool ignoreTransform: false, bool highQuality: false})