It'd be useful to have a standard way in the SDK to extend Map. This way, it can be reused across SDK and community packages. It'd also be useful to specify which type of map you want to use. For instance:

class ExtendableMap<K, V> implements Map {
  Map<K, V> _map = new HashMap();

  ExtendableMap();

  ExtendableMap.use(this._map);

  void operator []=(K key, V value) {
    _map[key] = value;
  }
}

Then in subclasses:

class CustomMap extends ExtendableMap {
  CustomMap() : super.use(new LinkedHashMap());
}

See discussion here: https://groups.google.com/a/dartlang.org/forum/#!topic/misc/Sj3guf3es24.