formatWithUnit_(quantity, durationUnit) {
    assertNumber(quantity, 'Quantity must be a number');
    assert(quantity >= 0, 'Duration value should not be less than zero.');

    // TODO(user): Add formatNative_ method when available.
    return this.formatPolyfill_(quantity, durationUnit);
  }
Formats a string with the amount and one unit.
@param {number} quantity  A value for the duration time.
@param {string} durationUnit  string such as hours, years,
    months.
@return {string} The formatted result.
@private