Incorporating review feedback from https://codereview.chromium.org/24294002/

Source/bindings/dart/custom/DartDocumentCustom.cpp

 // Copyright 2011, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //    * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //    * Redistributions in binary form must reproduce the above
@@ skipping 70 matching lines @@
             CustomElementCallbackDispatcher::CallbackDeliveryScope deliveryScope;
 
             const AtomicString& typeExtensionString = typeExtension;
             if (typeExtensionString.isNull()) {
                 result = receiver->createElement(localName, es);
             } else {
                 result = receiver->createElement(localName, typeExtensionString, es);
             }
         }
 
-        Dart_SetReturnValue(args, DartElement::toDart(result));
+        DartElement::returnToDart(args, result);
         if (es.hadException()) {
             exception = DartDOMWrapper::exceptionCodeToDartException(es);
             goto fail;
         }
         return;
     }
 
 fail:
     Dart_ThrowException(exception);
     ASSERT_NOT_REACHED();
@@ skipping 23 matching lines @@
             CustomElementCallbackDispatcher::CallbackDeliveryScope deliveryScope;
 
             const AtomicString& typeExtensionString = typeExtension;
             if (typeExtensionString.isNull()) {
                 result = receiver->createElementNS(namespaceURI, qualifiedName, es);
             } else {
                 result = receiver->createElementNS(namespaceURI, qualifiedName, typeExtensionString, es);
             }
         }
 
-        Dart_SetReturnValue(args, DartElement::toDart(result));
+        DartElement::returnToDart(args, result);
         if (es.hadException()) {
             exception = DartDOMWrapper::exceptionCodeToDartException(es);
             goto fail;
         }
         return;
     }
 
 fail:
     Dart_ThrowException(exception);
     ASSERT_NOT_REACHED();
 }
 
 } // namespace DartDocumentInternal
 
 } // namespace WebCore