code: function MapIterator(keys, valueGetter) {
      this._keys = keys;
      this._valueGetter = valueGetter;
      this._length = this._keys.length;
      this._index = -1;
      this._done = false;
  }
docs: Constructor of MapIterator<br>
  Creates iterator object with new keyword.
  @constructor
  @param  {Array} keys - The array of keys in the map
  @param  {function} valueGetter - Function that returns certain value,
  taking key and keyIndex as arguments.
  @ignore