If all you do is one DOM transformation then jquery will seem huge compared to just doing it by hand (probably hundreds or thousands of lines of code vs 3).
I guess at this point I'm just repeating the OP, but this is simply in no way demonstrative of Dart. It can at best serve as a learning tool of the internals (and perhaps there are valid criticisms once you look at those), but any conclusion on code size is very flawed from this one example.
However, in the case of jQuery the productivity / KB ratio is freakishly huge.
Refactoring tools? Some people would like that, but it still won't be bullet proof (as I understand the static types in Dart are optional), and there's nothing worse that a tool on which you can't rely on (personally I hate the Intellisense attempts being made in IDEs for Ruby or Python).
(That said, this thread is just silly. I don't like Dart because it looks like it's built for obsolescense, but the hello world extrapolation is nonsense, as has been pointed out.)