I expect an increasingly larger C/C++ codebase to be ported with emscripten and asm.js with decent enough performance (especially dev tools). Currently C++ code compiled with emscripten runs in Firefox only 1.5x slower than natively compiled code [1]. Notable ports can be found at emscripten wiki page [2].

[1] https://hacks.mozilla.org/2013/12/gap-between-asm-js-and-nat...

[2] https://github.com/kripken/emscripten/wiki