message: Add ImmutableMap#removeMany
diff: @@ -39,6 +39,14 @@ export default class ImmutableMap<K, V> {
  this._data = this._data.remove(key);
  }
  + removeMany(keys: K[]): void {
  + let data = this._data.beginMutation();
  + keys.forEach(key => {
  + data.remove(key);
  + });
  + this._data = data.endMutation();
  + }
  +
  has(key: K): boolean {
  return this.get(key) !== undefined;
  }
  