_resolveReferences(node) {
    node.obj.$traverse((obj) => this._resolveReferencesForObject(obj));
  }
The nodes inside the subgraph we are about to recursively upsert may
have references outside that graph that won't be available during the
recursive upsertGraph call. This method resolves the references.

TODO: This doesn't work if a recursively upserted node refers to
      a node inside another recursively upsertable graph.