Dart 1.0 M1 (first milestone) will be available soon, almost one year later after the public presentation of Dart to the world.
Dart is more and more stable from the language specification point of view, and still evolves from the other points (libraries, tools, optimizations, compilation, js generation, ...). Up to now, I was not completely convinced by this new google experiment, and I was still considering GWT as the best alternative to develop complex and efficient web applications, even more with the appearance of mgwt for the mobile market.
I have continued to take a glance on the project nevertheless, and what I observe is that this project is getting better and better, and more an more people seem involved inside Google on the project. Unfortunately the community is still not large, especially if we compare with Go, the other Google project (http://godashboard.appspot.com/project).
Dart future is unsure, but can be said on the right path, and certainly would simplify the web development even on the mobile side. It can be worth to go deeper inside it.