I'm not even thinking about "how it's going to get built" when I write test-first. I'm just writing (test) code as a client that's calling an API. It just so happens that the API (method, or method + object) in question doesn't actually exist yet, so the first thing I do is let the IDE generate them so it compiles.

Once the code compiles I go into the implementation and make it work.

Now and then I realize during implementation that I need some additional parameter, but it's easy to add that.