insert method
Inserts the object at position index in this list.
This increases the length of the list by one and shifts all objects at or after the index 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 insert(int index, E element) {
throw new UnsupportedError("Cannot add to immutable List.");
}