get(key) {
    if (!this.map_.has(key)) {
      return null;
    }
    // slice() clones the list so that it and the map can each be modified
    // without affecting the other.
    return this.map_.get(key).slice();
  }
Get a list of values by key.
@param {string} key
@return {Array<T>} or null if no such key exists.