ActionScript is really the easy part of making a Flash player: the hard part is being bug-compatible with almost every version of Flash ever shipped (the Adobe implementation has lots of branches depending on version of Flash that created the file!).
Video playback can be done via WebGL shaders, just like rendering video on the good old days with SIMD.
And most important, why not?
Someone will eventually do it, there are already JVM and CLR ongoing ports.