I can't speak to the overall progress (they've implemented a bunch of stuff I know there are big missing features). As for `yield`, I spoke with the developers a few months ago and they said that it's one of the next big features they'll work on. Don't expect it soon though, I bet it will take a while.

Considering that ES6 has pretty much been approved, and is expected to be finalized in about a year, we will definitely see this in most browsers in a year or two. It's a little ways off, but it's definitely happening.