operator [] method
The object at the given index in the list.
The index must be a valid index of this list,
which means that index must be non-negative and
less than length.
Implementation
@override
E operator [](int index) {
  if (source.length != _initialSize) {
    throw ConcurrentModificationError(source);
  }
  RangeError.checkValidIndex(index, this, null, length);
  return source[start + index];
}