[wip] Lint the javascript code with eslint

[fredj]

Display errors with gulp lint

work in progress

[BLamy]

Should probably be called gulp eslint as I would expect lint would be reserved for polylint once they fit that into the toolchain.

[fredj]

http://jscs.info/ should probably used instead as it's already the linter used in https://github.com/PolymerElements/polymer-starter-kit

[masonlouchart]

👍 for following the validation rules of the PSK. If you prefer ESLint maybe you could propose it for the PSK then add the eslint task to Polymer.

[dfreedm]

Thanks for the PR!

I'm marking this as needs investigation and/or docs to make sure we evaluate linters for the best use case.

Is the community collecting around eslint the best, or is jshint, xo, jscs, etc. the best?

[dfreedm]

Alright, after reviewing other linters in #3368, I think eslint is the way to go!

I have a few tweaks I want to make on this PR, so I'm going to merge it into a different branch and close this one. I'll reference it when it's ready.

Thanks again!