As I understand it, dart isn't trying to be a faster javascript. Its trying to be a better GWT.

Many googlers I've spoken to firmly believe that you need a strongly typed, OO language with good tooling to be able to develop any large application. They believe web apps are no exception.

Google has worked hard adding types to javascript with the closure compiler. But, there are limits to what you can achieve using javascript.

Comfortable programming in the large is the main value proposition of Dart. Expect to see good developer tools, debuggers, profilers and server-side runtime environments for Dart appearing over the next few years.

(Disclaimer: I don't share this view)