containsEntry(key, value) {
    const values = this.map_.get(key);
    if (!values) {
      return false;
    }

    const index = values.findIndex(function(v) {
      'use strict';
      return Object.is(v, value);
    });
    return index >= 0;
  }
@param {K} key The key to check.
@param {V} value The value to check.
@return {boolean} Whether the (key, value) pair exists in the multimap.