A language that compiles to JavaScript really needs better browser support. Decent web apps use feature detection to provide graceful degradation for browsers from the stone age. Dart breaks this by making it impossible to run any code at all.
I mean, "IE9 support coming soon!", what the hell? What about IE6? This isn't a CSS style or an animation library we're talking about.
The lack of browser support implies architectural choices that will always make old browsers lag behind. It means that the generated JS uses modern JS features. If it would be easy to support older browsers, they would've done so already - at least to some extent. The fact that it compiles only to JS for the very latest versions of a small number of browsers gives me the impression that a lot of language/compilation choices are closely tied to these new JS features. This, in turn, implies that supporting older JS versions is non-trivial.
Or maybe they've only been working on this thing for a few months and were slavishly fixing bugs right up to the announcement, and IE6 support wasn't really a priority?
Except for Isolates -> WebWorkers, there's not really anything that can't be emulated in IE6. GWT has to handle a significantly more complex set of semantics and still works on IE6.
But personally, I prefer if there wasn't IE6 support, or any non-HTML5 browser support for that matter. If you're going to do a new VM, do a clean-break, because legacy support is another factor which makes web development a pain.
Google officially started phasing out support for IE6 in January 2010. It's reasonable to assume that new languages (etc.) released by Google would be aligned with their general policy on which browsers are to be supported and which are not.
I mean, "IE9 support coming soon!", what the hell? What about IE6? This isn't a CSS style or an animation library we're talking about.