code: @Override
    public void process(Node externs, Node root) {
      checkState(topLevelStatements.isEmpty(), "process() called more than once");
      NodeTraversal t = new NodeTraversal(compiler, this, scopeCreator);
      t.traverseRoots(externs, root);
    }
docs: Convenience method for running this pass over a tree with this
  class as a callback.