message: Add ImmutableMap#clear
diff: @@ -15,6 +15,10 @@ export default class ImmutableMap<K, V> {
  return this._data.size;
  }
  + clear() {
  + this._data = new HAMTMap();
  + }
  +
  get(key: K): V {
  return this._data.get(key);
  }
  