Add 'at' to Array.prototype[@@unscopables]

Bug: v8:10961
Change-Id: I3746dca570de005d203a2648dcffedd81122f215
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2553157
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Gus Caplan <snek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71377}

src/init/bootstrapper.cc

@@ -4406,6 +4406,12 @@ void Genesis::InitializeGlobal_harmony_relative_indexing_methods() {
 
     SimpleInstallFunction(isolate(), array_prototype, "at",
                           Builtins::kArrayPrototypeAt, 1, true);
+
+    Handle<JSObject> unscopables = Handle<JSObject>::cast(
+        JSReceiver::GetProperty(isolate(), array_prototype,
+                                factory()->unscopables_symbol())
+            .ToHandleChecked());
+    InstallTrueValuedProperty(isolate(), unscopables, "at");
   }
 
   {

test/mjsunit/harmony/relative-indexing-methods.js

@@ -45,3 +45,5 @@ assertThrows(() => {
     a.at(0);
   }, TypeError);
 }
+
+assertEquals(Array.prototype[Symbol.unscopables].at, true);