>The client doesn't even know I'm doing it in Dart. I'm shipping the minified js file which works on all modern browsers because the Dart team set out to bring jQuery up to a higher level.

I would feel very uncomfortable about that unless it's the sort of client that doesn't really have the tech background to assume you're writing it in JS in the first place. I think if someone is paying to have software written for them, they have a right to have some control over what language it's written in and what libraries it uses. After all, if they need to hire someone else to maintain it or add to it later on, they're going to have a far easier time finding a JS developer to do that than a Dart developer (at least for now).

And, of course, you have the right to charge twice as much to do it in JS if it's going to take you twice as long.

I agree with you, but this client really doesn't care, only that it is done quickly and it works. They're pretty happy and are lining up more work for me. I'd say the greatest experience I had with it besides real types, code completion and intellisense, is that it ran first time on iPad (Safari, Chrome, FF), Mac and Windows FF and Chrome. I had a slight problem with IE9 and 10, but nothing too hard to fix. The client had paid someone to do it in ActionScript/Flash but the project had to be cancelled when Flash on mobile was mostly deprecated. I didn't look at the ActionScript code, but later found out there is a lib for that (haven't tried it and won't unless I have to someday) http://www.dartflash.com/