vue-cli/packages/@vue/cli-plugin-babel/index.js

const path = require('path')

module.exports = api => {
  api.chainWebpack(webpackConfig => {
    webpackConfig.module
      .rule('js')
        .test(/\.jsx?$/)
        .include
          .add(filepath => {
            const nodeModules = `${path.sep}node_modules${path.sep}`
            // For anything outside node_modules
            if (filepath.indexOf(nodeModules) === -1) {
              return true
            }
            // transpile webpack-dev-server client to ensure it works in older
            // browsers.
            // https://github.com/webpack/webpack-dev-server/pull/1241
            if (filepath.indexOf(`${nodeModules}webpack-dev-server${path.sep}client`) > -1) {
              return true
            }
            return false
          })
          .end()
        .use('babel-loader')
          .loader(require.resolve('babel-loader'))
  })
}