Fast-path conversion of Temporal.ZonedDateTime to Temporal.Instant

If passing a Temporal.ZonedDateTime where a Temporal.Instant is expected,
then create a Temporal.Instant directly from the ZonedDateTime's
[[Nanoseconds]] internal slot.
Previously, the ZonedDateTime's toString() method would have been called,
and the result would have been parsed into an Instant.

See: #1428

polyfill/lib/ecmascript.mjs

@@ -1273,6 +1273,10 @@ export const ES = ObjectAssign({}, ES2020, {
   },
   ToTemporalInstant: (item) => {
     if (ES.IsTemporalInstant(item)) return item;
+    if (ES.IsTemporalZonedDateTime(item)) {
+      const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
+      return new TemporalInstant(GetSlot(item, EPOCHNANOSECONDS));
+    }
     const ns = ES.ParseTemporalInstant(ES.ToString(item));
     const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
     return new TemporalInstant(ns);

spec/instant.html

@@ -519,8 +519,11 @@ <h1>ToTemporalInstant ( _item_ )</h1>
         The abstract operation ToTemporalInstant returns its argument _item_ if it is already a Temporal.Instant instance, converts _item_ to a new Temporal.Instant instance if possible, and throws otherwise.
       </p>
       <emu-alg>
-        1. If Type(_item_) is Object and _item_ has an [[InitializedTemporalInstant]] internal slot, then
-          1. Return _item_.
+        1. If Type(_item_) is Object, then
+          1. If _item_ has an [[InitializedTemporalInstant]] internal slot, then
+            1. Return _item_.
+          1. If _item_ has an [[InitializedTemporalZonedDateTime]] internal slot, then
+            1. Return ! CreateTemporalInstant(_item_.[[Nanoseconds]]).
         1. Let _string_ be ? ToString(_item_).
         1. Let _epochNanoseconds_ be ? ParseTemporalInstant(_string_).
         1. Return ? CreateTemporalInstant(_epochNanoseconds_).