Check that AllocationMementos never get scavengend.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index 880885593abe2916cdcad30bc46a369454fd663f..ae108d1e6d1186b5e261d15e64e3fe8ea149aa7e 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -525,6 +525,8 @@ void Heap::ScavengeObject(HeapObject** p, HeapObject* object) {
     return;
   }
 
+  // TODO(hpayer): temporary debugging code for issue 284577.
+  CHECK(object->map() != object->GetHeap()->allocation_memento_map());
   // Call the slow part of scavenge object.
   return ScavengeObjectSlow(p, object);
 }