The only problem I have with this sort of development effort is that it fails to recognize that Javascript is an awesome language unto itself and has no need of "fixing" by introducing a completely new language that is completely different.
Sure, it needs a few refinements here and there, but by and large Javascript is an incredible, flexible, and battle tested language. As a web developer with 11 years of experience, who has worked professionally with RoR, Java, PHP, MySQL, Oracle, and PostgreSQL, my feedback is just stop.
The world doesn't need another tool that obviates the need for engineers to learn Javascript to program on the web. Seriously, Javascript isn't that difficult. Just learn it, buckle down and spend a couple months writing Javascript and you'll be just fine.
If half the time spent writing Dart had been spent learning Javascript instead, then you would think in prototypes instead of classes, anonymous functions instead of one off objects, callbacks instead of um... not having callbacks, and dynamic variables instead of casting, casting, casting.
I'm not claiming Javascript is better then your language of choice, I'm just saying that as a language, it's awesome. It's not broke, and as a web developer I'm not looking for something to replace it.