There’s very little public information yet about Dart (nee, Dash), and as I’m not on Lars’ team I can’t comment about it.
[Several Google employees] and I attend TC39 meetings, work on implementations, and try to push JS forward in good faith. And boy, does it need a push.
Whatever you might think about programming languages for the browser, let me assure you of one thing: your problem isn’t the language. Not really, anyway. We’ve made good progress in the last year repairing some of the seams between JS and DOM ... But it’s only the beginning. The DOM is in terrible shape, and not due to implementation differences.