code: function on(thing, evt, fn) {
        if (thing.attachEvent) {
          thing.attachEvent('on'+ evt, fn);
        } else if (thing.addEventListener) {
          thing.addEventListener(evt, fn, false);
        } else {
          thing['on'+ evt] = fn;
        }
  
        return { on: on };
      }
docs: AddListener polyfill
  
  @param {Mixed} thing What ever we want to listen on.
  @param {String} evt The event we're listening for.
  @param {Function} fn The function that gets executed.
  @api private