Updating game.js to ES6

[lucasrafaldini]

Replacing empty named functions by ES6's arrow functions

[thomaspark]

Hey @lucasrafaldini, thanks for the PR.

We have take care replacing functions with arrow functions because there are several places within the functions where this is used, and this change will affect their values. See: https://stackoverflow.com/questions/31095710/methods-in-es6-objects-using-arrow-functions

We can keep functions as is, or also replace this with game.