Fix issue 6264 with a test case....

src/runtime.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 325 matching lines @@
   } else {
     RUNTIME_ASSERT(offset < ObjectTemplateInfo::kSize);
   }
   return *HeapObject::RawField(templ, offset);
 }
 
 
 static Object* Runtime_DisableAccessChecks(Arguments args) {
   ASSERT(args.length() == 1);
   CONVERT_CHECKED(HeapObject, object, args[0]);
-  bool needs_access_checks = object->map()->is_access_check_needed();
-  object->map()->set_is_access_check_needed(false);
+  Map* old_map = object->map();
+  bool needs_access_checks = old_map->is_access_check_needed();
+  if (needs_access_checks) {
+    // Copy map so it won't interfere constructor's initial map.
+    Object* new_map = old_map->CopyDropTransitions();
+    if (new_map->IsFailure()) return new_map;
+
+    Map::cast(new_map)->set_is_access_check_needed(false);
+    object->set_map(Map::cast(new_map));
+  }
   return needs_access_checks ? Heap::true_value() : Heap::false_value();
 }
 
 
 static Object* Runtime_EnableAccessChecks(Arguments args) {
   ASSERT(args.length() == 1);
   CONVERT_CHECKED(HeapObject, object, args[0]);
-  object->map()->set_is_access_check_needed(true);
+  Map* old_map = object->map();
+  if (!old_map->is_access_check_needed()) {
+    // Copy map so it won't interfere constructor's initial map.
+    Object* new_map = old_map->CopyDropTransitions();
+    if (new_map->IsFailure()) return new_map;
+
+    Map::cast(new_map)->set_is_access_check_needed(true);
+    object->set_map(Map::cast(new_map));
+  }
   return Heap::undefined_value();
 }
 
 
 static Object* ThrowRedeclarationError(const char* type, Handle<String> name) {
   HandleScope scope;
   Handle<Object> type_handle = Factory::NewStringFromAscii(CStrVector(type));
   Handle<Object> args[2] = { type_handle, name };
   Handle<Object> error =
       Factory::NewTypeError("redeclaration", HandleVector(args, 2));
@@ skipping 5562 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     Counters::gc_last_resort_from_js.Increment();
     Heap::CollectAllGarbage();
   }
 }
 
 
 } }  // namespace v8::internal