dart:collection

putIfAbsent method

dynamic putIfAbsent(Map map, key, dynamic ifAbsent())

Source

static putIfAbsent(Map map, key, ifAbsent()) {
  if (map.containsKey(key)) {
    return map[key];
  }
  final v = ifAbsent();
  map[key] = v;
  return v;
}