getEntries() {
    const keys = this.getKeys();
    const entries = [];
    for (let i = 0; i < keys.length; i++) {
      const key = keys[i];
      const values = this.get(key);
      for (let j = 0; j < values.length; j++) {
        entries.push([key, values[j]]);
      }
    }
    return entries;
  }
@return {!Array<!Array<K|V>>} An array of entries. Each entry is of the
    form [key, value].