Use Dart_InstanceIsSubtypeOf to check if instance is a direct subtype of specified type.

Source/bindings/dart/custom/DartXMLHttpRequestCustom.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 116 matching lines @@
 {
     Dart_Handle exception = 0;
     {
         XMLHttpRequest* receiver = DartDOMWrapper::receiver<XMLHttpRequest>(args);
 
         DartExceptionState es;
         DartDOMData* domData = static_cast<DartDOMData*>(Dart_GetNativeIsolateData(args));
         Dart_Handle arg1 = Dart_GetNativeArgument(args, 1);
         if (Dart_IsNull(arg1)) {
             receiver->send(es);
-        } else if (DartDOMWrapper::instanceOf<DartDocument>(domData, arg1)) {
+        } else if (DartDOMWrapper::subtypeOf<DartDocument>(domData, arg1)) {
             Document* asDocument = DartDocument::toNative(arg1, exception);
             if (exception)
                 goto fail;
             receiver->send(asDocument, es);
-        } else if (DartDOMWrapper::instanceOf<DartBlob>(domData, arg1)) {
+        } else if (DartDOMWrapper::subtypeOf<DartBlob>(domData, arg1)) {
             Blob* asBlob = DartBlob::toNative(arg1, exception);
             if (exception)
                 goto fail;
             receiver->send(asBlob, es);
-        } else if (DartDOMWrapper::instanceOf<DartFormData>(domData, arg1)) {
+        } else if (DartDOMWrapper::subtypeOf<DartFormData>(domData, arg1)) {
             DOMFormData* asDOMFormData = DartFormData::toNative(arg1, exception);
             if (exception)
                 goto fail;
             receiver->send(asDOMFormData, es);
         } else if (DartUtilities::isTypedData(arg1)) {
             RefPtr<ArrayBuffer> asArrayBuffer = DartUtilities::dartToArrayBuffer(arg1, exception);
             if (exception)
                 goto fail;
             receiver->send(asArrayBuffer.get(), es);
         } else {
@@ skipping 106 matching lines @@
     }
 
 fail:
     Dart_ThrowException(exception);
     ASSERT_NOT_REACHED();
 }
 
 } // DartXMLHttpRequestInternal
 
 } // WebCore