Previously getISOFields() was used to get the exact value of the
[[Calendar]] and [[TimeZone]] internal slots, as well as to get the
reference ISO year for PlainMonthDay and reference ISO day for
PlainYearMonth.
Use calendarId and timeZoneId for the former and toString() for the
latter.
We did this inconsistently in PlainDate and ZonedDateTime, and not in the
other calendar-carrying types. Additionally, we don't have to create the
calendar explicitly in PlainDate.
Each from() method except Calendar and TimeZone should test that when you
pass an instance of that type, the return value is a clone of that
instance, and a distinct object.
These tests existed already for some types; regularize them and add the
ones that didn't exist yet.
In order to test the referenceISODay of a PlainYearMonth we add an
argument to TemporalHelpers.assertPlainYearMonth.
Philip Chimento