Sky: Add a CustomDart attribute to the IDL compiler, and use that with Canvas

sky/engine/core/painting/Canvas.idl

Index: sky/engine/core/painting/Canvas.idl
diff --git a/sky/engine/core/painting/Canvas.idl b/sky/engine/core/painting/Canvas.idl
index 2eead032dc4beb5dcee3643ad3d1354dec3aa4e5..8aef9c562b3241af5a7e2c03ca7f40fc29d5c70a 100644
--- a/sky/engine/core/painting/Canvas.idl
+++ b/sky/engine/core/painting/Canvas.idl
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// TODO(mpcomplete): Figure out a better SkMatrix/SkRect representation.
-interface Canvas {
+// TODO(mpcomplete): Figure out a better SkMatrix representation.
+[CustomDart] interface Canvas {
   // Height and width are used for culling optimizations and do not necessarily
   // imply that the Canvas is backed by a buffer with any specific bounds.
   readonly attribute float height;