Well, then you have to carefully vet every JITting assembly language. Microsoft in fact does this: the WinRT APIs run on .NET, which has an associated assembly language CIL (formerly MSIL) and JITting runtime, the CLR.
So, if you want a JIT via a typed assembly language you can compile down to CIL. Firefox could do this, but then they couldn't specialize their JIT to run JavaScript properly, which is the hard part anyways. V8 and Gecko are very cool engines, and compiling javascript to CIL wouldn't compete.
So, if you want a JIT via a typed assembly language you can compile down to CIL. Firefox could do this, but then they couldn't specialize their JIT to run JavaScript properly, which is the hard part anyways. V8 and Gecko are very cool engines, and compiling javascript to CIL wouldn't compete.