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确保安装过babel-loader和babel-polyfill

cnpm i babel-loader babel-polyfill -D

将webpack配置文件修改为

entry: {
    app:['babel-polyfill', './src/main.js']
}

{
    test: /\.js$/,
    loader: 'babel-loader?cacheDirectory',
    include: [
        resolve('src'),
        resolve('test'),
        resolve('node_modules/_element-ui@2.4.7@element-ui/src'),
        resolve('node_modules/_element-ui@2.4.7@element-ui/package')
    ]
}

// 上述element-ui文件目录以本地 node_modules 中 element-ui 文件夹名称为准

重新打包即可