I just read Felgall say in another thread that browsers will stop what they are doing while loading a JS script. Then again, calling 200 separate files means 200 separate handshakes with the server.
If you mean, negotiation (some pages need some scripts and others don't), I have seen a dynamic loader as a solution to large scripts, where each page has a little script that calls what's needed for that page from the libs elsewhere... that way, the page who only needs 5 of the files only asks for 5, while the page than needs all 200 asks for those (caching should play a part here too).
I'll see if I can find it. It might have been part of one of the JS libraries.