This is an excellent idea, and is the goal of a project that Google announced at JSConf 2011 with much fanfare (some of it from Brendan Eich himself), Traceur: http://code.google.com/p/traceur-compiler/

However, Traceur has gained little traction. With the announcement of Dart, it's clear that Google's heart isn't really into the project.

See also: http://stackoverflow.com/questions/6506519/ecmascriptharmony...

Why don't we spend effort on building an ES:Harmony -> ES3 compiler rather then a CoffeeScript -> ES3 compiler.

It's more worthwhile future facing project.