formatPolyfill_(quantity, durationUnit) {
    /**
     * Find the right data based on unit, quantity, and plural.
     */
    const unitStyleString = this.getIcuFormattingPattern_(durationUnit);
    if (!unitStyleString) return '';

    /**
     * Formatter for the messages requiring units. Plural formatting needed.
     * @type {?MessageFormat}
     */
    // Take basic message and wrap with plural message type.
    const msgFormatter =
        new MessageFormat('{DURATION_VALUE,plural,' + unitStyleString + '}');
    return msgFormatter.format({'DURATION_VALUE': quantity});
  }
Format using pure JavaScript
@param {number} quantity Duration time value.
@param {string} durationUnit  string such as hours, years,
    months.
@return {string} The formatted result. May be empty string for an
  unsupported locale.
@private