Ensure correct enumeration indices in the dict

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index fe0683e047b5bc1da46b8c3506da8c333247ca42..a7d6faa5e6ca06a8c0e53662dd58bfa1a22a27f1 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -487,11 +487,20 @@ MaybeObject* JSObject::SetNormalizedProperty(String* name,
     set_properties(StringDictionary::cast(dict));
     return value;
   }
-  // Preserve enumeration index.
+
+  PropertyDetails original_details = property_dictionary()->DetailsAt(entry);
+  int enumeration_index;
+  // Preserve the enumeration index unless the property was deleted.
+  if (original_details.IsDeleted()) {
+    enumeration_index = property_dictionary()->NextEnumerationIndex();
+    property_dictionary()->SetNextEnumerationIndex(enumeration_index + 1);
+  } else {
+    enumeration_index = original_details.dictionary_index();
+    ASSERT(enumeration_index > 0);
+  }
+
   details = PropertyDetails(
-      details.attributes(),
-      details.type(),
-      property_dictionary()->DetailsAt(entry).dictionary_index());
+      details.attributes(), details.type(), enumeration_index);
 
   if (IsGlobalObject()) {
     JSGlobalPropertyCell* cell =