code: function CollectionIterator(keys, values, nextFn) {
      var that = this;
  
      this._keys   = keys;
      this._values = values;
      this["next"] = nextFn;
      this._cursor = -1;
  
      if (global["Symbol"]) { // ES2015 Spec
          this[global["Symbol"]["iterator"]] = function() {
              return that;
          };
      }
  }
docs: === CollectionIterator ==================================