operator []= method
- int index,
- E value
override
    Sets the value at the given index in the list to value.
The index must be a valid index of this list,
which means that index must be non-negative and
less than length.
Implementation
@override
void operator []=(int index, E value) {
  if (index < 0 || index >= length) {
    throw RangeError('Index $index must be in the range [0..$length).');
  }
  _table[(_head + index) & (_table.length - 1)] = value;
}