removeN method
Removes the first n occurrences of item (or last if n is negative).
Implementation
void removeN(T item, int n) {
if (n == 0) return;
if (n > 0) {
for (var i = 0; i < n; i++) {
removeFirst(item);
}
} else {
for (var i = 0; i < -n; i++) {
removeLastOccurrence(item);
}
}
}