Normative: Change Calendar.prototype.toJSON not to call toString

The toJSON method should return the value in the internal slot, instead of
making an observable call to toString.

See: #1808

UPSTREAM_COMMIT=9199c05fdd58ddf45a03211396762e57da4fe6bc

polyfill/lib/calendar.mjs

@@ -250,7 +250,7 @@ export class Calendar {
   }
   toJSON() {
     if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
-    return ES.ToString(this);
+    return GetSlot(this, CALENDAR_ID);
   }
   static from(item) {
     const calendarSlotValue = ES.ToTemporalCalendarSlotValue(item);