Mentioned in http://www.aminutewithbrendan.com/pages/20101122.

JS is becoming the compiler target "bytecode", without needing verifiers or new and complex standards to be adopted by multiple browser vendors.

JS has gaps as a target language, for sure. We are working on filling them (e.g. 64-bit and other int types).

These are easy bugs to fix compared to creating a new, portable, and future-friendly bytecode standard in addition to keeping up with a competitive JS engine. Since JS is incumbent, it's hard for any browser vendor to justify a new thing with zero users at first, and too much risk of non-standardization or JVML-like albatross status.