Because we can now compile almost *anything* into Javascript, and the ability will only increase, I don't think it's a problem.
Everyone is now targetting Javascript - Adobe Flash, we have LLVM (enscripten). And Mozilla's "asm.js" - which is just an extension of Javascript (actually, erm, a reduced set of JS, but with an efficient native compiler) means that we might be able to approach (in some cases) native performance using Javascript.
So I think that all the other things - Dart, NaCl etc, are just distractions, all roads appear to lead to Javascript. Although we're all using js under the covers, many of us won't be writing much of it however.