JS JITS *are* in fact among the fastest in dynamic languages, matched only by LuaJIT.
If Java or Python would JIT into JS, thereby reusing the JS JIT, then they could be very fast in the browser too. I've been wanting to do that exact project with PyPy (compile PyPy C code using Emscripten, add a JS JIT backend), am still hoping to interest the PyPy devs in it some day.
For Flash specifically though, the language is very close to JS. JITing into JS there is simpler than the alternatives, and should give good results. Would be interesting to compare to the AS engines in the other FOSS Flash implementations.