They've done it twice. MotionTwin did a lot of work in AS2 so they built a better compiler for it (MTASC), which compiled faster and made faster swfs (it's still used in production at the BBC).
Then when AS3 came out they seem to have thought "Oh come on!" given the quality of the compiler and language features so they built a better language. Again, faster and better compilation, as well as properly implemented things like generics (particularly compared to the awful implementation in AS3).
It's a lovely little language, and cross compiles to a vast range of platforms with ease. I built an android app which ran a webserver to control a TV app built in it (compiled to JS). I also had good success compiling it to PHP (cross-platform serialisation & deserialisation turned out to be easier and more performant than parsing XML on some low power devices).
MotionTwin made my working day so much better, not having to use the CS4 was wonderful.
Actually, HaXe, Neko, mtasc and MotionTwin projects of one person: Nicolas Cannasse. Dude is a legend, and community surrounding those projects is amazing.
Great example of this; adobe released alchemy[1] a couple years back with which you could compile very basic c/c++ code to a "swic" library which could then be referenced in flash. Although initially free to try adobe wanted to turn this into a premium feature[2]. It was quickly reverse engineered and turned into a haxe api, that made the step of using basic c/c++ and compiling it to a swic obsolete, keeping everything nicely in the haxe ecosystem[3]. If that was not impressive enough they did the same thing with "pixelbender3d"[4] and created hxsl[5].
