Removes all pairs from the map.
After this, the map is empty.
void clear() {
int len = _map.length;
if (hasObservers && len > 0) {
_map.forEach((key, value) {
notifyChange(new MapChangeRecord.remove(key, value));
});
notifyPropertyChange(#length, len, 0);
_notifyKeysValuesChanged();
}
_map.clear();
}