public void addNodeById(Node node)
        {
            if (idMap == null)
                idMap = new HashMap<String, Node>();

            idMap.put(node.id, node);
        }
Test ser/deser of polymorphic graph objects (references and cyclic dependencies).

@author David Yu
@created Jan 19, 2011
protostuff-runtime/src/test/java/io/protostuff/runtime/PolymorphicRuntimeGraphTest.java