### Version 3.0.0-rc.3 ### Reproduction link [https://webpack.js.org/configuration/dev-server/#devserver-before](https://webpack.js.org/configuration/dev-server/#devserver-before) ### Steps to reproduce Add: ``` devServer: { before(app, server) { server.sockWrite(server.sockets, "content-changed"); } } ``` to `vue.config.js` file. ### What is expected? sockWrite method on server is hit successfully ### What is actually happening? `server` is undefined <!-- generated by vue-issues. DO NOT REMOVE -->