insertAll method Null safety
override
Inserts all objects of iterable at position index in this list.
This increases the length of the list by the length of iterable and
shifts all later objects towards the end of the list.
The list must be growable.
The index value must be non-negative and no greater than length.
Implementation
void insertAll(int index, Iterable<E> iterable) {
throw new UnsupportedError("Cannot add to immutable List.");
}