Dart can produce JavaScript that is faster http://news.dartlang.org/2013/03/why-dart2js-produces-faster...
The JavaScript output of Dart2js works across all modern browsers (IE9 and above, among others.) No browser hacks required.
Having said that, I wish they would stop working on advancing Dart and ship a 1.0 version. It's been 1.5 years since it was announced. Make the advancements in the 1.1 or 2.0 and give the tool developers a solid target to optimize for in 1.0. Let us not have to sell clients on a beta development system, even though that's what I'm doing and it's working, despite some panic when Dart developers break things with breaking changes (which is often.)