Don't overwrite transitions in SetPropertyIgnoreAttributes.

src/objects-inl.h

 // Copyright 2012 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 4762 matching lines @@
 }
 
 
 void Map::set_transitions(TransitionArray* transition_array,
                           WriteBarrierMode mode) {
   // Transition arrays are not shared. When one is replaced, it should not
   // keep referenced objects alive, so we zap it.
   // When there is another reference to the array somewhere (e.g. a handle),
   // not zapping turns from a waste of memory into a source of crashes.
   if (HasTransitionArray()) {
+#ifdef DEBUG
+    for (int i = 0; i < transitions()->number_of_transitions(); i++) {
+      Map* target = transitions()->GetTarget(i);
+      if (target->instance_descriptors() == instance_descriptors()) {
+        Name* key = transitions()->GetKey(i);
+        int new_target_index = transition_array->Search(key);
+        ASSERT(new_target_index != TransitionArray::kNotFound);
+        ASSERT(transition_array->GetTarget(new_target_index) == target);
+      }
+    }
+#endif
     ASSERT(transitions() != transition_array);
     ZapTransitions();
   }
 
   WRITE_FIELD(this, kTransitionsOrBackPointerOffset, transition_array);
   CONDITIONAL_WRITE_BARRIER(
       GetHeap(), this, kTransitionsOrBackPointerOffset, transition_array, mode);
 }
 
 
@@ skipping 2001 matching lines @@
 #undef READ_UINT32_FIELD
 #undef WRITE_UINT32_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_