PlainMonthDay({month: Infinity, day}) handling

The previous commit failed the`PlainMonthDay({month: Infinity, day})`
test because 'MInfinity' is obviously invalid. This commit adds a Symbol
that can signal to the ISO calendar implementation that both  `calendar`
and `monthCode` were omitted from the inputs. This differentiates from
the case where the calendar is explicitly ISO (not omitted) because in
that case `monthCode` is still required.

polyfill/lib/calendar.mjs

@@ -199,6 +199,15 @@ impl['iso8601'] = {
     if (fields.month !== undefined && fields.year === undefined && fields.monthCode === undefined) {
       throw new TypeError('either year or monthCode required with month');
     }
+    // If the user omits the calendar from `from`, then it's clearly not
+    // intended to be cross-calendar code because the user is unambiguously
+    // asking for the ISO calendar. Therefore there's no requirement for
+    // `monthCode` to be present. ES.UndefinedMonthCode is a symbol that is only
+    // passed in the "omits the calendar and omits monthCode" case. This is
+    // needed to differentiate from the case where the user specified the ISO
+    // calendar explicitly. In that case, `monthCode` is required and
+    // `undefined` will throw above.
+    if (fields.monthCode === ES.UndefinedMonthCode) fields.monthCode = undefined;
     fields = resolveNonLunisolarMonth(fields);
     let { month, day } = fields;
     ({ month, day } = ES.RegulateMonthDay(month, day, overflow));

polyfill/lib/ecmascript.mjs

@@ -999,11 +999,11 @@ export const ES = ObjectAssign({}, ES2020, {
     for (const fieldRecord of fields) {
       const [property, defaultValue] = fieldRecord;
       let value = bag[property];
-      if (value === undefined) {
+      if (value === undefined || value === ES.UndefinedMonthCode) {
         if (fieldRecord.length === 1) {
           throw new TypeError(`required property '${property}' missing or undefined`);
         }
-        value = defaultValue;
+        if (value === undefined) value = defaultValue;
       } else {
         any = true;
         if (BUILTIN_CASTS.has(property)) {
@@ -1270,6 +1270,7 @@ export const ES = ObjectAssign({}, ES2020, {
     if (!ES.IsTemporalInstant(result)) throw new TypeError('invalid result');
     return result;
   },
+  UndefinedMonthCode: Symbol('undefined-month-code'),
   ToTemporalMonthDay: (item, constructor, options = {}) => {
     if (ES.Type(item) === 'Object') {
       if (ES.IsTemporalMonthDay(item)) return item;
@@ -1280,7 +1281,7 @@ export const ES = ObjectAssign({}, ES2020, {
       const fieldNames = ES.CalendarFields(calendar, ['day', 'month', 'monthCode', 'year']);
       const fields = ES.ToTemporalMonthDayFields(item, fieldNames);
       if (calendarAbsent && fields.month !== undefined && fields.monthCode === undefined) {
-        fields.monthCode = `M${ES.ToString(fields.month)}`;
+        fields.monthCode = ES.UndefinedMonthCode;
       }
       return ES.MonthDayFromFields(calendar, fields, constructor, options);
     }

polyfill/test/intl.mjs

@@ -882,6 +882,9 @@ describe('Intl', () => {
         if (test.monthCode === undefined) test.monthCode = `M${test.month}`;
         const { calendar, monthCode, month, day, year, isoReferenceYear } = test;
         const md = Temporal.PlainMonthDay.from({ year, month, day, calendar });
+        const isoString = md.toString();
+        const mdFromIso = Temporal.PlainMonthDay.from(isoString);
+        equal(mdFromIso, md);
         const isoFields = md.getISOFields();
         equal(md.monthCode, monthCode);
         equal(md.day, day);