Do not mark prototype transitions link as weak in heap snapshot.

src/heap-snapshot-generator.cc

 // Copyright 2013 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 1304 matching lines @@
 
 
 void V8HeapExplorer::ExtractMapReferences(int entry, Map* map) {
   if (map->HasTransitionArray()) {
     TransitionArray* transitions = map->transitions();
     int transitions_entry = GetEntry(transitions)->index();
     Object* back_pointer = transitions->back_pointer_storage();
     TagObject(back_pointer, "(back pointer)");
     SetInternalReference(transitions, transitions_entry,
                          "back_pointer", back_pointer);
+
+    if (FLAG_collect_maps && map->CanTransition()) {
+      if (!transitions->IsSimpleTransition()) {
+        if (transitions->HasPrototypeTransitions()) {
+          FixedArray* prototype_transitions =
+              transitions->GetPrototypeTransitions();
+          MarkAsWeakContainer(prototype_transitions);
+          TagObject(prototype_transitions, "(prototype transitions");
+          SetInternalReference(transitions, transitions_entry,
+                               "prototype_transitions", prototype_transitions);
+        }
+        // TODO(alph): transitions keys are strong links.
+        MarkAsWeakContainer(transitions);
+      }
+    }
+
     TagObject(transitions, "(transition array)");
-    MarkAsWeakContainer(transitions);
     SetInternalReference(map, entry,
                          "transitions", transitions,
                          Map::kTransitionsOrBackPointerOffset);
   } else {
     Object* back_pointer = map->GetBackPointer();
     TagObject(back_pointer, "(back pointer)");
     SetInternalReference(map, entry,
                          "back_pointer", back_pointer,
                          Map::kTransitionsOrBackPointerOffset);
   }
@@ skipping 1809 matching lines @@
   writer_->AddString("\"<dummy>\"");
   for (int i = 1; i < sorted_strings.length(); ++i) {
     writer_->AddCharacter(',');
     SerializeString(sorted_strings[i]);
     if (writer_->aborted()) return;
   }
 }
 
 
 } }  // namespace v8::internal