Editorial: Remove dead code from ZonedDateTime.prototype.toLocaleString

This removes some steps that can throw and therefore look observable, but
they were effectively dead code; the value stored in the [[TimeZone]]
internal slot is guaranteed to be a valid available time zone identifier.

h/t Anba

Closes: #3057

polyfill/lib/zoneddatetime.mjs

@@ -391,14 +391,10 @@ export class ZonedDateTime {
       // The rest of the defaults will be filled in by formatting the Instant
     }
 
-    const timeZoneIdentifier = GetSlot(this, TIME_ZONE);
-    if (ES.IsOffsetTimeZoneIdentifier(timeZoneIdentifier)) {
+    optionsCopy.timeZone = GetSlot(this, TIME_ZONE);
+    if (ES.IsOffsetTimeZoneIdentifier(optionsCopy.timeZone)) {
       // Note: https://github.com/tc39/ecma402/issues/683 will remove this
       throw new RangeErrorCtor('toLocaleString does not currently support offset time zones');
-    } else {
-      const record = ES.GetAvailableNamedTimeZoneIdentifier(timeZoneIdentifier);
-      if (!record) throw new RangeErrorCtor(`toLocaleString formats built-in time zones, not ${timeZoneIdentifier}`);
-      optionsCopy.timeZone = record.identifier;
     }
 
     const formatter = new DateTimeFormat(locales, optionsCopy);

spec/intl.html

@@ -1782,15 +1782,7 @@ <h1>Temporal.ZonedDateTime.prototype.toLocaleString ( [ _locales_ [ , _options_
           <emu-alg>
             1. Let _zonedDateTime_ be the *this* value.
             1. Perform ? RequireInternalSlot(_zonedDateTime_, [[InitializedTemporalZonedDateTime]]).
-            1. Let _timeZone_ be _zonedDateTime_.[[TimeZone]].
-            1. Let _timeZoneParseResult_ be ? ParseTimeZoneIdentifier(_timeZone_).
-            1. If _timeZoneParseResult_.[[OffsetMinutes]] is ~empty~, then
-              1. Let _timeZoneIdentifierRecord_ be GetAvailableNamedTimeZoneIdentifier(_timeZone_).
-              1. If _timeZoneIdentifierRecord_ is ~empty~, throw a *RangeError* exception.
-              1. Set _timeZone_ to _timeZoneIdentifierRecord_.[[Identifier]].
-            1. Else,
-              1. Set _timeZone_ to FormatOffsetTimeZoneIdentifier(_timeZoneParseResult_.[[OffsetMinutes]]).
-            1. Let _dateTimeFormat_ be ? CreateDateTimeFormat(%Intl.DateTimeFormat%, _locales_, _options_, ~any~, ~all~, _timeZone_).
+            1. Let _dateTimeFormat_ be ? CreateDateTimeFormat(%Intl.DateTimeFormat%, _locales_, _options_, ~any~, ~all~, _zonedDateTime_.[[TimeZone]]).
             1. If _zonedDateTime_.[[Calendar]] is not *"iso8601"* and CalendarEquals(_zonedDateTime_.[[Calendar]], _dateTimeFormat_.[[Calendar]]) is *false*, then
               1. Throw a *RangeError* exception.
             1. Let _instant_ be ! CreateTemporalInstant(_zonedDateTime_.[[EpochNanoseconds]]).