applyTo method
Actually applies this operation on the list
by mutating it.
Implementation
void applyTo(List<Item> list) {
if (isInsertion) {
list.insert(index, item);
} else {
assert(
list[index] == item,
"Tried to remove item $item at index $index, but there's a different "
'item at that position: ${list[index]}.',
);
list.removeAt(index);
}
}