constructor(locale, firstDate, secondDate, pattern, expected) {
    this.locale = locale;
    this.firstDate = firstDate;
    this.secondDate = secondDate;
    this.pattern = pattern;
    this.expected = expected;
  }
@param {string} locale
@param {!Array<number>} firstDate
@param {!Array<number>} secondDate
@param {number|!dateIntervalSymbols.DateIntervalPatternMap} pattern
@param {string} expected