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

Source/bindings/dart/custom/DartBlobCustom.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 62 matching lines @@
 
         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)) {
                 RefPtr<ArrayBuffer> arrayBuffer = DartUtilities::dartToArrayBuffer(item, exception);
                 if (exception)
                     goto fail;
                 ASSERT(arrayBuffer);
                 blobBuilder.append(arrayBuffer.get());
-            } else if (DartDOMWrapper::instanceOf<DartBlob>(domData, item)) {
+            } else if (DartDOMWrapper::subtypeOf<DartBlob>(domData, item)) {
                 Blob* blob = DartBlob::toNative(item, exception);
                 if (exception)
                     goto fail;
                 ASSERT(blob);
                 blobBuilder.append(blob);
             } else {
                 String stringValue = DartUtilities::dartToString(item, exception);
                 if (exception)
                     goto fail;
                 blobBuilder.append(stringValue, endings);
@@ skipping 17 matching lines @@
     if (!blob)
         return Dart_Null();
 
     if (blob->isFile())
         return DartFile::createWrapper(static_cast<File*>(blob));
 
     return DartDOMWrapper::createWrapper<DartBlob>(blob);
 }
 
 }