isDefined(node) {
    if ('ident' == node.nodeName) {
      return new nodes.Boolean(this.lookup(node.name));
    } else {
      throw new Error('invalid "is defined" check on non-variable ' + node);
    }
  }
Check if the given `node` is an ident, and if it is defined.

@param {Node} node
@return {Boolean}
@api private