Remove the extensibility flag. Instead just rely on hidden_string as indication.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 27e10e09fa6b2eb90ece8241a85487422d2798dd..0492ac4a8a53145f73a3ad3382661770a2a60109 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -5041,7 +5041,7 @@ RUNTIME_FUNCTION(Runtime_DefineDataPropertyUnchecked) {
     ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
         isolate, result,
         JSObject::SetOwnPropertyIgnoreAttributes(
-            js_object, name, obj_value, attr, PERFORM_EXTENSIBILITY_CHECK,
+            js_object, name, obj_value, attr,
             JSReceiver::MAY_BE_STORE_FROM_KEYED, JSObject::DONT_FORCE_FIELD));
     return *result;
   }
@@ -5195,9 +5195,8 @@ MaybeHandle<Object> Runtime::DefineObjectProperty(
                                   SLOPPY, false, DEFINE_PROPERTY);
     } else {
       if (name->IsString()) name = String::Flatten(Handle<String>::cast(name));
-      return JSObject::SetOwnPropertyIgnoreAttributes(
-          js_object, name, value, attr, PERFORM_EXTENSIBILITY_CHECK,
-          store_from_keyed);
+      return JSObject::SetOwnPropertyIgnoreAttributes(js_object, name, value,
+                                                      attr, store_from_keyed);
     }
   }
 
@@ -5211,9 +5210,8 @@ MaybeHandle<Object> Runtime::DefineObjectProperty(
     return JSObject::SetElement(js_object, index, value, attr,
                                 SLOPPY, false, DEFINE_PROPERTY);
   } else {
-    return JSObject::SetOwnPropertyIgnoreAttributes(
-        js_object, name, value, attr, PERFORM_EXTENSIBILITY_CHECK,
-        store_from_keyed);
+    return JSObject::SetOwnPropertyIgnoreAttributes(js_object, name, value,
+                                                    attr, store_from_keyed);
   }
 }