get(key) {
    const values = this.map_.get(key);
    return values ? goog.array.clone(values) : [];
  }
Gets the values correspond to the given key.
@param {K} key The key to retrieve.
@return {!Array<V>} An array of values corresponding to the given
    key. May be empty. Note that the ordering of values are not
    guaranteed to be consistent.