I don't quite see how to do this; I tried 

```cpp
auto x = FunctionTemplate::New(node_isolate, dataWrapperTmpl);
x->InstanceTemplate()->SetInternalFieldCount(1); 
````

but there is no appropriate overload for `FunctionTemplate::New`. Did you mean the result of `Local<FunctionTemplate>::New`, i.e. just create an intermediate variable instead of using `->` immediately?