Checking syntax used

[domenic]

This is an easy enhancement: as a prefilter, before the transpiler, verify that they're using ES6 syntax, and not cheating by using ES5.

This could be done either with quick regexes, e.g. check that spread or rest is present by searching for ..., or it could be more robust by checking the AST. We'd probably have to use the Traceur AST. I am not sure what the state of the documentation for that is.