Add a native subtype of function that uses the native C++ class hierarchy to check if an element

Source/bindings/dart/custom/DartDOMStringListCustom.cpp

Index: Source/bindings/dart/custom/DartDOMStringListCustom.cpp
===================================================================
--- Source/bindings/dart/custom/DartDOMStringListCustom.cpp	(revision 1437)
+++ Source/bindings/dart/custom/DartDOMStringListCustom.cpp	(working copy)
@@ -11,9 +11,10 @@
 
 PassRefPtr<DOMStringList> DartDOMStringList::toNative(Dart_Handle handle, Dart_Handle& exception)
 {
-    DartDOMData* domData = DartDOMData::current();
-    if (DartDOMWrapper::instanceOf<DartDOMStringList>(domData, handle))
+    if (DartDOMWrapper::subtypeOf(handle, DartDOMStringList::dartClassId)) {
+        DartDOMData* domData = DartDOMData::current();
         return DartDOMWrapper::unwrapDartWrapper<DartDOMStringList>(domData, handle, exception);
+    }
 
     Vector<Dart_Handle> elements;
     DartUtilities::extractListElements(handle, exception, elements);