Allow alternative era for Chinese calendar

Year 1 in the Chinese calendar corresponds to 2637 BCE in ICU4X.

See <https://docs.rs/icu/latest/icu/calendar/chinese/struct.Chinese.html#year-and-era-codes>.

test/intl402/Temporal/PlainDate/prototype/until/until-across-lunisolar-leap-months.js

@@ -9,8 +9,18 @@ features: [Temporal]
 
 // 2001 is a leap year in the Chinese calendar with a M04L leap month.
 // Therefore, month: 6 is M05 in 2001 but M06 in 2000 which is not a leap year.
-const one = Temporal.PlainDate.from({ year: 2000, month: 6, day: 1, calendar: 'chinese' });
-const two = Temporal.PlainDate.from({ year: 2001, month: 6, day: 1, calendar: 'chinese' });
+
+const year2000 = new Temporal.PlainDate(2000, 3, 1).withCalendar("chinese").year;
+const year2001 = new Temporal.PlainDate(2001, 3, 1).withCalendar("chinese").year;
+
+const one = Temporal.PlainDate.from({ year: year2000, month: 6, day: 1, calendar: 'chinese' });
+const two = Temporal.PlainDate.from({ year: year2001, month: 6, day: 1, calendar: 'chinese' });
+
+assert.sameValue(one.inLeapYear, false, "year 2000 is not a leap year");
+assert.sameValue(one.monthCode, "M06", "sixth month in year 2000 has month code M06");
+
+assert.sameValue(two.inLeapYear, true, "year 2001 is a leap year");
+assert.sameValue(two.monthCode, "M05", "sixth month in year 2001 has month code M05");
 
 const expected = { years: 'P12M', months: 'P12M', weeks: 'P50W4D', days: 'P354D' };