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)
It isn't trying to be a javascript, but it's certainly trying to be faster. Performance features prominently in every statement of their design goals. Not only that, but performance in all modern browsers - i.e. including the ones they don't control and won't ship with a Dart VM. I don't see how they can possibly pull that off. Can anybody?
Dart might be many things, none of them come remotely close to _strongly typed_.
Also, I never understood why Google ever started GWT, the idea itself is like a bad Apr 1st joke that got out of hand. I guess that the legions of Java programmers google has been hiring have started to take over sigh