Add stackTrace to Error object.

sdk/lib/json/json.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart.json;
 
 // JSON parsing and serialization.
 
 /**
  * Error thrown by JSON serialization if an object cannot be serialized.
  *
  * The [unsupportedObject] field holds that object that failed to be serialized.
  *
  * If an object isn't directly serializable, the serializer calls the 'toJson'
  * method on the object. If that call fails, the error will be stored in the
  * [cause] field. If the call returns an object that isn't directly
  * serializable, the [cause] will be null.
  */
-class JsonUnsupportedObjectError implements Error {
+class JsonUnsupportedObjectError extends Error {
   /** The object that could not be serialized. */
   final unsupportedObject;
   /** The exception thrown by object's [:toJson:] method, if any. */
   final cause;
 
   JsonUnsupportedObjectError(this.unsupportedObject, { this.cause });
 
   String toString() {
     if (cause != null) {
       return "Calling toJson method on object failed.";
@@ skipping 787 matching lines @@
         first = false;
       });
       sb.write('}');
       seen.removeLast();
       return true;
     } else {
       return false;
     }
   }
 }