[builtins] take slow path in IsConcatSpreadable if proxy in prototype

src/objects.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/objects.h"
 
 #include <cmath>
 #include <iomanip>
 #include <sstream>
 
@@ skipping 18968 matching lines @@
   // Might happen for the debug context.
   if (!constructor->shared()->IsApiFunction()) return nullptr;
 
   Object* data_obj =
       constructor->shared()->get_api_func_data()->access_check_info();
   if (data_obj->IsUndefined(isolate)) return nullptr;
 
   return AccessCheckInfo::cast(data_obj);
 }
 
+bool JSReceiver::HasProxyInPrototype(Isolate* isolate) {
+  for (PrototypeIterator iter(isolate, this, kStartAtReceiver,
+                              PrototypeIterator::END_AT_NULL);
+       !iter.IsAtEnd(); iter.AdvanceIgnoringProxies()) {
+    if (iter.GetCurrent<Object>()->IsJSProxy()) return true;
+  }
+  return false;
+}
+
 }  // namespace internal
 }  // namespace v8