removeAll(key) {
    // We have to first retrieve the values from the backing map because
    // we need to keep track of count (and correctly calculates the
    // return value). values may be undefined.
    const values = this.map_.get(key);
    if (this.map_.delete(key)) {
      this.count_ -= values.length;
      return true;
    }

    return false;
  }
Removes all values corresponding to the given key.
@param {K} key The key whose values are to be removed.
@return {boolean} Whether any value is removed.