Although, I will say that there's no problem with fragmentation of Chrome or Firefox (okay, maybe a startup or two trying to fork Firefox), and for those we get nightly builds and real-time access to their source code repository.
Whatever Google's reason for not letting out real-time source code, it's got nothing to do with market uniformity. My guess is that it's instead about trying to kow tow to hardware vendors trying to keep things under wraps, like new hardware features for which Google might be building suitable drivers or applications.