Add support for lists and backward references when decoding a message to a Dart_CObject object

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index 9389e81601e6a5c5e11f005707c24bbd9864aa89..a71e8cae03560e6b52c5a206e4c06f95d09ca168 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -449,12 +449,16 @@ class CMessageReader : public BaseReader {
   Dart_CObject* ReadObjectImpl(intptr_t header);
   Dart_CObject* ReadIndexedObject(intptr_t object_id);
 
+  // Add object to backward references.
+  void AddBackwardReference(intptr_t id, Dart_CObject* obj);
+
   Dart_CObject_Internal* AsInternal(Dart_CObject* object) {
     ASSERT(object->type >= Dart_CObject::kNumberOfTypes);
     return reinterpret_cast<Dart_CObject_Internal*>(object);
   }
 
   ReAlloc alloc_;
+  GrowableArray<Dart_CObject*> backward_references_;
 
   Dart_CObject type_arguments_marker;
   Dart_CObject dynamic_type_marker;