I realize subclassing Array in the past was a no-no, but it works perfectly well with CoffeeScript, and has been quite useful and performant.

Is there a way 6to5 could allow subclassing Array (and other built-ins?). NOT at the cost of the beautiful code generation .. but if its just that browser foo doesn't support, I'd be OK with that. And as I recall, es6 explicitly says we can subclass builtins.