_len is not defined

babel config

{
  "presets": [
    "es2015"
  ],
  "plugins": [
    "react-hot-loader/babel"
  ]
}

original.js

export function spread(...args) {
  return args.push(1)
}

transpiled.js

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.spread = spread;
function spread() {
  for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key++) {
    args[_key] = arguments[_key];
  }

  return args.push(1);
}

(function () {
  function tagSource(fn, localName) {
    if (typeof fn !== "function") {
      return;
    }

    if (fn.hasOwnProperty("__source")) {
      return;
    }

    try {
      Object.defineProperty(fn, "__source", {
        enumerable: false,
        configurable: true,
        value: {
          fileName: "abc.js",
          localName: localName
        }
      });
    } catch (err) {}
  }

  tagSource(spread, "spread");
  tagSource(_len, "_len");
  tagSource(args, "args");
  tagSource(_key, "_key");
})();

I'm having the same issue at tagSource(_len, '_len');, where _len is only defined in a for loop.