插件/预设文件不允许导出对象，只允许导出函数。在[...]/babel-preset-es2015/lib/index.js中

module.exports = {
  presets: [
    // presets configuration
  ],
  plugins: [
    // plugins configuration
  ]
};

module.exports = function() {
  return {
    presets: [
      // presets configuration
    ],
    plugins: [
      // plugins configuration
    ]
  };
};