Adds value
to the end of this list,
extending the length by one.
Throws an UnsupportedError
if the list is fixed-length.
void add(E value) {
int len = _list.length;
_notifyChangeLength(len, len + 1);
if (hasListObservers) {
_recordChange(new ListChangeRecord(this, len, addedCount: 1));
}
_list.add(value);
}