Fast-path conversion of Temporal.PlainDateTime to Temporal.PlainTime

If passing a Temporal.PlainDateTime where a Temporal.PlainTime is
expected, then create a Temporal.PlainTime directly from the
PlainDateTime's [[ISOHour]], [[ISOMinute]], [[ISOSecond]],
[[ISOMillisecond]], [[ISOMicrosecond]], and [[ISONanosecond]] internal
slots.
Previously, the PlainDateTime's hour, minute, second, millisecond,
microsecond, nanosecond, and calendar getters would have been called.

See: #1428

polyfill/lib/ecmascript.mjs

@@ -1340,6 +1340,17 @@ export const ES = ObjectAssign({}, ES2020, {
     let hour, minute, second, millisecond, microsecond, nanosecond, calendar;
     if (ES.Type(item) === 'Object') {
       if (ES.IsTemporalTime(item)) return item;
+      if (ES.IsTemporalDateTime(item)) {
+        const TemporalPlainTime = GetIntrinsic('%Temporal.PlainTime%');
+        return new TemporalPlainTime(
+          GetSlot(item, ISO_HOUR),
+          GetSlot(item, ISO_MINUTE),
+          GetSlot(item, ISO_SECOND),
+          GetSlot(item, ISO_MILLISECOND),
+          GetSlot(item, ISO_MICROSECOND),
+          GetSlot(item, ISO_NANOSECOND)
+        );
+      }
       calendar = item.calendar;
       if (calendar) {
         calendar = ES.ToTemporalCalendar(calendar);

spec/plaintime.html

@@ -610,6 +610,8 @@ <h1>ToTemporalTime ( _item_ [ , _overflow_ ] )</h1>
         1. If Type(_item_) is Object, then
           1. If _item_ has an [[InitializedTemporalTime]] internal slot, then
             1. Return _item_.
+          1. If _item_ has an [[InitializedTemporalDateTime]] internal slot, then
+            1. Return ! CreateTemporalTime(_item_.[[ISOHour]], _item_.[[ISOMinute]], _item_.[[ISOSecond]], _item_.[[ISOMillisecond]], _item_.[[ISOMicrosecond]], _item_.[[ISONanosecond]]).
           1. Let _calendar_ be ? Get(_item_, *"calendar"*).
           1. If _calendar_ is not *undefined*, then
             1. Set _calendar_ to ? ToTemporalCalendar(_calendar_).