insertAll method
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.
An error occurs if the index is less than 0 or greater than length.
An UnsupportedError occurs if the list is fixed-length.
Implementation
void insertAll(int index, Iterable<E> iterable) {
throw new UnsupportedError("Cannot add to immutable List.");
}