I used to work at a company called Spaceport.io (now defunct) where we had a nearly complete implementation. It included an impressive GPU-accelerated renderer and an optimising compiler from AS3 to JavaScript—we could run on mobile, desktop, and even WebGL, and do neat things like update over WiFi instead of USB during development.

It was intended for Flash game development, and worked pretty well for that purpose. Unfortunately, we spent a lot of time dealing with large customers who wanted to use it to port existing Flash games to mobile, which was basically a neverending treadmill of having to implement obscure features that they didn’t even know they were relying on. :/

Parts of it have been open-sourced, but I wish the parent company would just release the whole thing. Last I knew, there was one guy maintaining it for one customer after the main engineers were acquihired.