Now, my question is, to those who worked with either or both, what's the state of them. Are they used a lot? Are they usable? What's the community of them? Which one is best, or shows most promise according to you? And most importantly, what's the state of both, are they still actively developed, and which one do you think will survive the longest?
(Edit: as a bonus, I'm planning on making a little game in it, probably using the html5 canvas, so which one has best support for this?)
I'm the opposite of Eric J. Smith - I can only speak for Haxe.
For app development (not games), Haxe can still be good - this is more of what I do. I generally use pretty standard JS technologies like jQuery, but Haxe gives a whole bunch of nice extras with compile time error-checking, a really powerful language and the ability to share code with the server. I'm sure Dart provides some of the same advantages.
In terms of maturity, stability and support:
I would also love to hear from people who have used both as to differences in the actual languages. From what I've picked up Haxe seems to be more strictly typed (which I prefer - catch more errors at compile time!) but some people don't like that.