The problem is you are using ES6 syntax within your webpack.config.js file. For the config file you should only use ES5 because it will be executed by NodeJS which does not yet support import/export syntax.
Webpack doesn't transpile (via babel-loader) the config file; only the ./app/index.js entry point, as specified in the config file
P.S.: If you would also like to use ES6 in webpack files, you can use babel-register.