Speed up non-ISO calendar tests about 6x

I was wrong about what was making non-ISO calendars so slow. I
thought the problem was `formatToParts()`, but it turns out that the
`DateTimeFormat` constructor is really slow and also allocates
ridiculous amounts of RAM. See more details here:
https://bugs.chromium.org/p/v8/issues/detail?id=6528

@littledan in https://bugs.chromium.org/p/v8/issues/detail?id=6528#c4
recommended to cache DateTimeFormat instances, so that's what this
commit does.

The result is a 6x speedup in non-ISO calendar tests.
Before: 6398.83ms
After: 1062.26ms

A similar speedup is likely for `ES.GetCanonicalTimeZoneIdentifier`.
Caching time zone canonicalization (in a separate PR) should have
a big positive impact on ZonedDateTIme and TimeZone perf.

Many thanks to @fer22f for uncovering this optimization in
js-temporal/temporal-polyfill#7.

polyfill/lib/calendar.mjs

@@ -7,6 +7,10 @@ import { CALENDAR_ID, ISO_YEAR, ISO_MONTH, ISO_DAY, CreateSlots, GetSlot, HasSlo
 const ArrayIncludes = Array.prototype.includes;
 const ArrayPrototypePush = Array.prototype.push;
 const ObjectAssign = Object.assign;
+const ObjectEntries = Object.entries;
+const IntlDateTimeFormat = globalThis.Intl.DateTimeFormat;
+const MathAbs = Math.abs;
+const MathFloor = Math.floor;
 
 const impl = {};
 
@@ -427,19 +431,31 @@ function simpleDateDiff(one, two) {
  */
 const nonIsoHelperBase = {
   // The properties and methods below here should be the same for all lunar/lunisolar calendars.
+  getFormatter() {
+    // `new Intl.DateTimeFormat()` is amazingly slow and chews up RAM. Per
+    // https://bugs.chromium.org/p/v8/issues/detail?id=6528#c4, we cache one
+    // DateTimeFormat instance per calendar. Caching is lazy so we only pay for
+    // calendars that are used. Note that the nonIsoHelperBase object is spread
+    // into each each calendar's implementation before any cache is created, so
+    // each calendar gets its own separate cached formatter.
+    if (typeof this.formatter === 'undefined') {
+      this.formatter = new IntlDateTimeFormat(`en-US-u-ca-${this.id}`, {
+        day: 'numeric',
+        month: 'numeric',
+        year: 'numeric',
+        era: this.eraLength,
+        timeZone: 'UTC'
+      });
+    }
+    return this.formatter;
+  },
   isoToCalendarDate(isoDate, cache) {
     let { year: isoYear, month: isoMonth, day: isoDay } = isoDate;
     const key = JSON.stringify({ func: 'isoToCalendarDate', isoYear, isoMonth, isoDay, id: this.id });
     const cached = cache.get(key);
     if (cached) return cached;
 
-    const dateTimeFormat = new Intl.DateTimeFormat(`en-US-u-ca-${this.id}`, {
-      day: 'numeric',
-      month: 'numeric',
-      year: 'numeric',
-      era: this.eraLength,
-      timeZone: 'UTC'
-    });
+    const dateTimeFormat = this.getFormatter();
     let parts, isoString;
     try {
       isoString = toUtcIsoDateString({ isoYear, isoMonth, isoDay });
@@ -763,7 +779,7 @@ const nonIsoHelperBase = {
   },
   addMonthsCalendar(calendarDate, months, overflow, cache) {
     const { day } = calendarDate;
-    for (let i = 0, absMonths = Math.abs(months); i < absMonths; i++) {
+    for (let i = 0, absMonths = MathAbs(months); i < absMonths; i++) {
       const days = months < 0 ? -this.daysInPreviousMonth(calendarDate, cache) : this.daysInMonth(calendarDate, cache);
       const isoDate = this.calendarToIsoDate(calendarDate, 'constrain', cache);
       const addedIso = this.addDaysIso(isoDate, days, cache);
@@ -970,7 +986,7 @@ const helperHebrew = ObjectAssign({}, nonIsoHelperBase, {
   minMaxMonthLength(calendarDate, minOrMax) {
     const { month, year } = calendarDate;
     const monthCode = this.getMonthCode(year, month);
-    const monthInfo = Object.entries(this.months).find((m) => m[1].monthCode === monthCode);
+    const monthInfo = ObjectEntries(this.months).find((m) => m[1].monthCode === monthCode);
     if (monthInfo === undefined) throw new RangeError(`unmatched Hebrew month: ${month}`);
     const daysInMonth = monthInfo[1].days;
     return typeof daysInMonth === 'number' ? daysInMonth : daysInMonth[minOrMax];
@@ -1096,7 +1112,7 @@ const helperIslamic = ObjectAssign({}, nonIsoHelperBase, {
   constantEra: 'ah',
   estimateIsoDate(calendarDate) {
     const { year } = this.adjustCalendarDate(calendarDate);
-    return { year: Math.floor((year * this.DAYS_PER_ISLAMIC_YEAR) / this.DAYS_PER_ISO_YEAR) + 622, month: 1, day: 1 };
+    return { year: MathFloor((year * this.DAYS_PER_ISLAMIC_YEAR) / this.DAYS_PER_ISO_YEAR) + 622, month: 1, day: 1 };
   }
 });
 
@@ -1587,7 +1603,7 @@ const helperChinese = ObjectAssign({}, nonIsoHelperBase, {
   calendarType: 'lunisolar',
   inLeapYear(calendarDate, cache) {
     const months = this.getMonthList(calendarDate.year, cache);
-    return Object.entries(months).length === 13;
+    return ObjectEntries(months).length === 13;
   },
   monthsInYear(calendarDate, cache) {
     return this.inLeapYear(calendarDate, cache) ? 13 : 12;
@@ -1601,14 +1617,7 @@ const helperChinese = ObjectAssign({}, nonIsoHelperBase, {
     const key = JSON.stringify({ func: 'getMonthList', calendarYear, id: this.id });
     const cached = cache.get(key);
     if (cached) return cached;
-    const dateTimeFormat = new Intl.DateTimeFormat(`en-US-u-ca-${this.id}`, {
-      day: 'numeric',
-      month: 'numeric',
-      year: 'numeric',
-      era: 'short',
-      timeZone: 'UTC'
-    });
-
+    const dateTimeFormat = this.getFormatter();
     const getCalendarDate = (isoYear, daysPastFeb1) => {
       const isoStringFeb1 = toUtcIsoDateString({ isoYear, isoMonth: 2, isoDay: 1 });
       const legacyDate = new Date(isoStringFeb1);
@@ -1723,7 +1732,7 @@ const helperChinese = ObjectAssign({}, nonIsoHelperBase, {
         }
       } else if (monthCode === undefined) {
         const months = this.getMonthList(year, cache);
-        const monthEntries = Object.entries(months);
+        const monthEntries = ObjectEntries(months);
         const largestMonth = monthEntries.length;
         if (overflow === 'reject') {
           ES.RejectToRange(month, 1, largestMonth);