Bug 1297722 - Fix ES6 use in production

[wlach]

• Use babel to make transpile ES6 code to ES5
• Add a trivial use of the => function to make sure this actually works
  in production

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This change is

[edmorley]

Could we just drop usage of uglify instead? By how much does it actually reduce the end JS files after gzip?
Alternatively, is there a newer version of it/alternative package that is ES6 compatible?
It would be great to see the number of build steps that delay every deploy go down rather than up if possible :-) (they are the long pole for Heroku deploys particularly, since Heroku does a better job of caching the npm deps)

[wlach]

Yeah, I tried both those options (especially an uglify replacement) and couldn't find a recipe that worked. :( As far as I can tell, the world of node.js packaging is a complete and utter mess: at this point I've spent about 3 hours on this problem which is probably already more than I can justify. Once an es6-compatible version of uglify is released, we can always switch off the babel step.