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

Source/bindings/dart/custom/DartBlobCustom.cpp

Index: Source/bindings/dart/custom/DartBlobCustom.cpp
===================================================================
--- Source/bindings/dart/custom/DartBlobCustom.cpp	(revision 1430)
+++ Source/bindings/dart/custom/DartBlobCustom.cpp	(working copy)
@@ -80,7 +80,7 @@
                     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;