operator [] method

Map operator [] (int index)

Returns the object at the given index in the list or throws a RangeError if index is out of bounds.

Implementation 

Map operator [](int index) {
  if (JS("bool", "# >>> 0 !== # || # >= #", index, index, index, length))
    throw new RangeError.index(index, this);
  return this.item(index);
}