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

Source/bindings/dart/custom/DartDocumentCustom.cpp

Index: Source/bindings/dart/custom/DartDocumentCustom.cpp
diff --git a/Source/bindings/dart/custom/DartDocumentCustom.cpp b/Source/bindings/dart/custom/DartDocumentCustom.cpp
index e3e4b8d69d94bb5681f52a370b84701a5f98495a..deab8b1e2051e7930d893aaed1efa38d1a615acf 100644
--- a/Source/bindings/dart/custom/DartDocumentCustom.cpp
+++ b/Source/bindings/dart/custom/DartDocumentCustom.cpp
@@ -88,7 +88,7 @@ void createElementCallback(Dart_NativeArguments args)
             }
         }
 
-        Dart_SetReturnValue(args, DartElement::toDart(result));
+        DartElement::returnToDart(args, result);
         if (es.hadException()) {
             exception = DartDOMWrapper::exceptionCodeToDartException(es);
             goto fail;
@@ -132,7 +132,7 @@ void createElementNSCallback(Dart_NativeArguments args)
             }
         }
 
-        Dart_SetReturnValue(args, DartElement::toDart(result));
+        DartElement::returnToDart(args, result);
         if (es.hadException()) {
             exception = DartDOMWrapper::exceptionCodeToDartException(es);
             goto fail;