Er, you might want to check that. Darwin hasn't been open source in a long time.
Yeah, not since OS X version 10.5.4
Oh wait, that's the current version.
The source, PPC and Intel (gasp! that's unpossible!) for Darwin can be found at Apple - Darwin - Releases
Perhaps you just need a bit more practice with this new "Google" thing. I'm sure you would have found it on your next search.