Dart is great and I like how they are focusing on making it fast and creating a special VM for it (dartium, although it compiles to javascript, too). It's tempting to target a compiler to generate dart code, too, instead of javascript. But there is one deal breaker for me - dart can't compile locally in the browser (even a dartium one). It requires a server or else compiling off-line (same issue with Closure Compiler, too). You can't code a dart app on a chromebook, for example (unless you use remote desktop or install ubuntu on it). The future to me is doing all development work in the browser, and only connecting to the server for the server components or version control, like with c9.io and other browser-based IDEs. Even Eclipse has a browser-based version now - Orion.

It's not a huge deal - there are over a hundred alternative languages that compile to javascript out there now, many of which do support compiling in the browser: https://github.com/jashkenas/coffee-script/wiki/List-of-lang...