Very active development; see https://www.youtube.com/playlist?list=PLOU2XLYxmsILKY-A1kq4e... for a few recent presentations about the progress.

(disclaimer: I work on the Dart team at Google)

To add to Kasper's reply, Dart is more active than it ever has before.

Part of that is just because of how versatile it is; it's not just a web programming language. It has multiple uses from mobile to web, back to front end. In fact, there's even a new OS where it's a first class citizen.

Watch some of the videos from the recent Dart summit and be amazed!

Yes it is. Google uses it internally with Angular Dart (a version of Angular made especially for Dart). And the Flutter project looks very interesting to built native mobile apps with Dart (instead of Java for example).