Sets offset in internal reference and internal storage to value.
This is just for completeness, but should never be used, because twig cannot set properties and should not.
@link http://php.net/manual/en/arrayaccess.offsetset.php
mixed $offset: The offset to assign the value to.
mixed $value: The value to set.
public function offsetSet($offset, $value) {
$this->writableRef[$offset] = $value;
parent::offsetSet($offset, $value);
}