Also mark as prototype when passing in while creating a function.

src/objects.cc

 // Copyright 2013 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/v8.h"
 
 #include "src/accessors.h"
 #include "src/allocation-site-scopes.h"
 #include "src/api.h"
 #include "src/arguments.h"
@@ skipping 7270 matching lines @@
   }
 
   Handle<Name> name = isolate->factory()->observed_symbol();
   ConnectTransition(map, new_map, name, FULL_TRANSITION);
 
   return new_map;
 }
 
 
 Handle<Map> Map::CopyAsPrototypeMap(Handle<Map> map) {
+  if (map->is_prototype_map()) return map;
   Handle<Map> result = Copy(map);
   result->mark_prototype_map();
   return result;
 }
 
 
 Handle<Map> Map::Copy(Handle<Map> map) {
   Handle<DescriptorArray> descriptors(map->instance_descriptors());
   int number_of_own_descriptors = map->NumberOfOwnDescriptors();
   Handle<DescriptorArray> new_descriptors =
@@ skipping 9749 matching lines @@
 #define ERROR_MESSAGES_TEXTS(C, T) T,
   static const char* error_messages_[] = {
       ERROR_MESSAGES_LIST(ERROR_MESSAGES_TEXTS)
   };
 #undef ERROR_MESSAGES_TEXTS
   return error_messages_[reason];
 }
 
 
 } }  // namespace v8::internal