Avoid setting transitions in-place for cached maps when observed

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index f28430430ff4060552b80d8c59bcd4214f4fe038..e1cdc625fb1b78e7af5521858e4019ff3c5c1f86 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -5816,7 +5816,7 @@ void JSObject::SetObserved(Handle<JSObject> object) {
   if (transition_index != TransitionArray::kNotFound) {
     new_map = handle(old_map->GetTransition(transition_index), isolate);
     ASSERT(new_map->is_observed());
-  } else if (old_map->CanHaveMoreTransitions()) {
+  } else if (object->HasFastProperties() && old_map->CanHaveMoreTransitions()) {
     new_map = Map::CopyForObserved(old_map);
   } else {
     new_map = Map::Copy(old_map);