Allow ES6 constructs
Allow ES6 constructs

These constructs are already in use in the ngeo dependency and nowadays
allows for cleaner and more maintanable code.

The closure-compiler transpiles ES6 to ES5 so it still run on all browsers.

Geoportail-Luxembourg/geoportailv3,Geoportail-Luxembourg/geoportailv3,Geoportail-Luxembourg/geoportailv3,Geoportail-Luxembourg/geoportailv3,Geoportail-Luxembourg/geoportailv3