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

Source/bindings/dart/custom/DartWebSocketCustom.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 36 matching lines @@
 
         DartDOMData* domData = static_cast<DartDOMData*>(Dart_GetNativeIsolateData(args));
         Dart_Handle dataHandle = Dart_GetNativeArgument(args, 1);
 
         DartExceptionState es;
         if (DartUtilities::isTypedData(dataHandle)) {
             RefPtr<ArrayBuffer> data = DartUtilities::dartToArrayBuffer(dataHandle, exception);
             if (exception)
                 goto fail;
             receiver->send(data.get(), es);
-        } else if (DartDOMWrapper::instanceOf<DartBlob>(domData, dataHandle)) {
+        } else if (DartDOMWrapper::subtypeOf<DartBlob>(domData, dataHandle)) {
             Blob* data = DartBlob::toNative(dataHandle, exception);
             if (exception)
                 goto fail;
             receiver->send(data, es);
         } else if (Dart_IsString(dataHandle)) {
             DartStringAdapter data = DartUtilities::dartToString(dataHandle, exception);
             if (exception)
                 goto fail;
             receiver->send(data, es);
         } else {
@@ skipping 113 matching lines @@
     }
 
 fail:
     Dart_ThrowException(exception);
     ASSERT_NOT_REACHED();
 }
 
 }
 
 }