Removes the object at position index from this list.
This method reduces the length of this by one and moves all later objects
down by one position.
Returns the removed object.
The index must be in the range 0 ≤ index < length.
Throws an UnsupportedError if this is a fixed-length list. In that case
the list is not modified.
Source
E removeAt(int pos) {
throw new UnsupportedError("Cannot remove from immutable List.");
}