The Closure Compiler supports compiling ES6 code already, and can also do transpilation down to ES3/ES5. [1][2]
Google also has the Traceur Compiler which does transpilation of new JS features (ES6, ES7 proposals, and a couple of features that aren't in either) into older versions. [3]
[1]: https://github.com/google/closure-compiler/wiki/Releases#oct...
[2]: https://github.com/google/closure-compiler/wiki/ECMAScript6#... (might be a bit out of date, since there have been releases since that document was last edited)