One thing that bothers me about Dart is that the Angular.js team decided to go with their own language AtScript, even though they were quite experienced with Dart in AngularDart.
Do they know something about the health of Dart that we don't?
AtScript makes decent sense in that it's ES6 plus other things they want to land in ES7. JavaScript is here to stay, they have to support it. Dart is also here to stay. It will be the answer to structured (typed) web apps and its server-side story is great and getting better by the day.
They're planning to use AtScript to transpile to ES6 and Dart by extending the ES6 transpiler called Traceur (source: https://docs.google.com/a/google.com/document/d/11YUzC-1d0V1...). The basic issue for Angular is they'd like to support users of ES5, ES6, and Dart with the same code base. That's currently a bit hard to do with a Dart code base (although a lot of us want to see that get better!)
disclaimer: I've worked on the Dart team and on the Traceur compiler in the past too. So I'm definitely not unbiased on these topics :)
Do they know something about the health of Dart that we don't?