That being said, traction for any new programming language is hard. A programmer's learning time is limited and most programmers would treat it as an investment. In Dart's case, the investment will make more sense if the Dart team could get the VM onto Chrome's release channel (not Dartium/Chromium as it currently stands). I bet that's going to be a hard fought battle, but I believe this would incentivize developers to code in Dart and serve Dart2Js generated js files to other browsers (FF, IE etc.); especially because the coding/debugging experience is so much better than vanilla javascript (I am now prepared for coffeescript advocates to join the fray).
As an aside there might be some App Engine integration that can help the cause. "Specifically, we are working on server-side Dart support for Managed VM" https://code.google.com/p/googleappengine/issues/detail?id=6...
There are so many novel, cool "JS replacements" that it's easy to feel some paralysis about the whole matter. Do you go with Dart? TypeScript? Clojure? Or do you stick with regular JS and start learning a new framework (e.g. Angular)?
They're not necessarily mutually exclusive, but I think a good majority of web developers are waiting to see what unfolds before committing one way or the other.
When I last looked into it, there had to be some strange stuff to be done, to call JS from Dart and Dart from JS.
It just didn't felt as naturally as in TypeScript or LiveScript.