add(key, value) {
    let values = this.map_.get(key);
    if (!values) {
      this.map_.set(key, (values = []));
    }

    values.push(value);
    this.count_++;
  }
Adds the given (key, value) pair to the map. The (key, value) pair
is guaranteed to be added.
@param {K} key The key to add.
@param {V} value The value to add.