Doesn't Google already have a web based, but internal only, IDE? I don't know if that'd be easy to make external, but my understanding is that they've got a lot of internal users on it.
Yeah, and Cider was getting surprisingly good at the point I left Google (I was initially a skeptic). But so much of what made it good came from its tight integration with other internal tooling. I'd be surprised if it's ever externalized in a form that captures most of that value.
[1]: http://thenextweb.com/dd/2016/07/14/amazon-buys-cloud9-aws/