an} True if |id| is a key in the dictionary.
   */
        valueExists(id) {
            return id in this.data_;
        }

        /**
   * Fetches a value, expecting that it exists.
   * @param {string} id The key that identifies the desired value.
   * @return {*} The corresponding value.
   */
        getValue(id) {
            expect(this.data_, 'No data. Did you remember to include strings.js?');
            const value = this.data_[id];
            expect(typeof value !== 'undefined', 'Could not fin