Languages like CoffeeScript do fall into the "alternative syntax" category. And things like emscripten (and some others I've seen) probably fall into the latter. If you try to abstract out a traditional VM on top of Javascript, you're likely to get poor results.
As another commenter suggests, though, we have a language like Haxe, which is much more than just an alternative syntax. And GWT does a very good job with Java -- with the exception of some corner cases, it's usually as fast as hand-written Javascript, and often faster because of a great deal of static optimization. The Google Closure compiler arguably defines a different language (if you turn on "advanced optimizations", it only accepts an effectively-statically-typed Javascript variant).
As another commenter suggests, though, we have a language like Haxe, which is much more than just an alternative syntax. And GWT does a very good job with Java -- with the exception of some corner cases, it's usually as fast as hand-written Javascript, and often faster because of a great deal of static optimization. The Google Closure compiler arguably defines a different language (if you turn on "advanced optimizations", it only accepts an effectively-statically-typed Javascript variant).