Firstly I think it's important to point out how much of what is currently being said is speculative. Most people's opinions on Dart are covered with disclaimers especially this one from Brendan Eich and this one is going to be no different :)
Colin made the point that Google might be onto a loser if they don't support jQuery, which he felt was unlikely to happen/be possible. I'd personally be very surprised if Dart didn't support jQuery, probably through some form of "native" interface. My guess is it'll be implemented in a similar way to the way Closure-Compiler. You supply an extern file which defines the jQuery methods annotated in the Closure convention. Once you have the extern file you can then reference the functions defined in it from Closure annotated code with full type checking etc..
I'd like to end with Brendan Eich's parting comment from the link above -
Since the '60s, researchers have dreamed of universal object files (ANDF, e.g.). It would be too funny if far-future JS merges into this single standard too!("too funny" in my experience means this will definitely happen...)/be