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

Source/bindings/dart/custom/DartXMLHttpRequestCustom.cpp

Index: Source/bindings/dart/custom/DartXMLHttpRequestCustom.cpp
===================================================================
--- Source/bindings/dart/custom/DartXMLHttpRequestCustom.cpp	(revision 1430)
+++ Source/bindings/dart/custom/DartXMLHttpRequestCustom.cpp	(working copy)
@@ -134,17 +134,17 @@
         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;