code: function preTransformVFor (el, options) {
    var exp = getAndRemoveAttr(el, 'v-for');
    if (!exp) {
      return
    }
  
    var res = parseFor(exp);
    if (!res) {
      if (process.env.NODE_ENV !== 'production' && options.warn) {
        options.warn(("Invalid v-for expression: " + exp));
      }
      return
    }
  
    var desc = {
      '@expression': res.for,
      '@alias': res.alias
    };
    if (res.iterator2) {
      desc['@key'] = res.iterator1;
      desc['@index'] = res.iterator2;
    } else {
      desc['@index'] = res.iterator1;
    }
  
    delete el.attrsMap['v-for'];
    addRawAttr(el, '[[repeat]]', desc);
  }
docs: /*