Polyfill: Add missing brand checks in TimeZone.

Author: Ms2ger

lib/timezone.mjs

@@ -54,6 +54,7 @@ export class TimeZone {
     return ES.GetIANATimeZoneOffsetNanoseconds(GetSlot(instant, EPOCHNANOSECONDS), id);
   }
   getOffsetStringFor(instant) {
+    if (!ES.IsTemporalTimeZone(this)) throw new TypeError('invalid receiver');
     instant = ES.ToTemporalInstant(instant);
     return ES.BuiltinTimeZoneGetOffsetStringFor(this, instant);
   }
@@ -63,6 +64,7 @@ export class TimeZone {
     return ES.BuiltinTimeZoneGetPlainDateTimeFor(this, instant, calendar);
   }
   getInstantFor(dateTime, options = undefined) {
+    if (!ES.IsTemporalTimeZone(this)) throw new TypeError('invalid receiver');
     dateTime = ES.ToTemporalDateTime(dateTime);
     options = ES.GetOptionsObject(options);
     const disambiguation = ES.ToTemporalDisambiguation(options);

test/usertimezone.mjs

@@ -94,20 +94,14 @@ describe('Userland time zone', () => {
     };
 
     const inst = Temporal.Instant.fromEpochNanoseconds(0n);
-    const dt = new Temporal.PlainDateTime(1976, 11, 18, 15, 23, 30, 123, 456, 789);
 
-    it('has offset string +00:00', () =>
-      equal(Temporal.TimeZone.prototype.getOffsetStringFor.call(obj, inst), '+00:00'));
     it('converts to DateTime', () => {
       equal(`${Temporal.TimeZone.prototype.getPlainDateTimeFor.call(obj, inst)}`, '1970-01-01T00:00:00');
       equal(
         `${Temporal.TimeZone.prototype.getPlainDateTimeFor.call(obj, inst, 'gregory')}`,
         '1970-01-01T00:00:00[u-ca=gregory]'
       );
     });
-    it('converts to Instant', () => {
-      equal(`${Temporal.TimeZone.prototype.getInstantFor.call(obj, dt)}`, '1976-11-18T15:23:30.123456789Z');
-    });
     it('offset prints in instant.toString', () => equal(inst.toString({ timeZone: obj }), '1970-01-01T00:00:00+00:00'));
     it('prints in zdt.toString', () => {
       const zdt = new Temporal.ZonedDateTime(0n, obj);