code: function createVisitor(handlers) {
    nodeTypes.forEach(function (nodeType) {
      if (typeof handlers[nodeType] === 'undefined') {
        throw new Error('No handler defined for ' + nodeType.displayName);
      }
    });
  
    return {
      /**
       * Call the given handler for this node type
       * @param  {Object} node    the AST node
       * @param  {Object} context context to pass through to handlers
       * @return {Object}
       */
      visit: function (node, context) {
        return this.handlers[node.displayName].call(this, node, context);
      },
      handlers: handlers
    };
  }
docs: Helper for creating visitors. Take an object of node name to handler
  mappings, returns an object with a "visit" method that can be called
  @param  {Object.<string,function(node,context)>} handlers A mapping of node
  type to visitor functions
  @return {{visit: function(node,context)}}  A visitor object with a "visit"
  method that can be called on a node with a context