Merged r19586 into 3.23 branch.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 0e6b9804e09fe6776b0b61d97362d1d3ecf223ab..07bcb7632c4809142e056da3843c2272a8d413ae 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -2594,6 +2594,7 @@ void MarkCompactCollector::ClearNonLivePrototypeTransitions(Map* map) {
     Object* prototype = prototype_transitions->get(proto_offset + i * step);
     Object* cached_map = prototype_transitions->get(map_offset + i * step);
     if (IsMarked(prototype) && IsMarked(cached_map)) {
+      ASSERT(!prototype->IsUndefined());
       int proto_index = proto_offset + new_number_of_transitions * step;
       int map_index = map_offset + new_number_of_transitions * step;
       if (new_number_of_transitions != i) {