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

Source/bindings/dart/custom/DartBlobCustom.cpp

Index: Source/bindings/dart/custom/DartBlobCustom.cpp
===================================================================
--- Source/bindings/dart/custom/DartBlobCustom.cpp	(revision 1437)
+++ Source/bindings/dart/custom/DartBlobCustom.cpp	(working copy)
@@ -71,7 +71,6 @@
 
         uint32_t length = blobParts.size();
 
-        DartDOMData* domData = static_cast<DartDOMData*>(Dart_GetNativeIsolateData(args));
         for (uint32_t i = 0; i < length; ++i) {
             Dart_Handle item = blobParts[i];
             if (DartUtilities::isTypedData(item)) {
@@ -80,7 +79,7 @@
                     goto fail;
                 ASSERT(arrayBuffer);
                 blobBuilder.append(arrayBuffer.get());
-            } else if (DartDOMWrapper::instanceOf<DartBlob>(domData, item)) {
+            } else if (DartDOMWrapper::subtypeOf(item, DartBlob::dartClassId)) {
                 Blob* blob = DartBlob::toNative(item, exception);
                 if (exception)
                     goto fail;