I'm trying to use an ES6 npm package (in this case one called gamesystem) in a project and build with webpack (and babel) it fails to build any ES6 code inside the external dependency, why is that? If I have the same code as a dependency with a relative path it works fine.

App code:

'use strict';
import World from 'gamesystem'; // Breaks
//import World from '../node_modules/gamesystem'; // Also breaks
//import World from '../gamesystem'; // Works (copied the same directory gamesystem outside the node_modules directory)
let world = new World();

Error:

ERROR in ./~/gamesystem/World.js
Module parse failed: /home/user/project/node_modules/gamesystem/World.js Line 3: Unexpected token
You may need an appropriate loader to handle this file type.
| 'use strict';
|
| import System from './System';
|
| export default class World {
 @ ./src/app.js 3:18-39

Webpack config:

'use strict';
// Modules
let WebpackNotifierPlugin = require('webpack-notifier');
let HtmlWebpackPlugin = require('html-webpack-plugin');
let config = {
  entry: {
    app: __dirname + '/../src/app.js'
  },
  devtool: 'source-map',
  devServer: {
    stats: {
      modules: false,
      cached: false,
      colors: true,
      chunk: false
    },
    proxy: require('./devProxy')
  },
  module: {
    loaders: [{
      test: /\.js$/,
      loader: 'babel',
      exclude: /node_modules/,
      query: {
        presets: ['es2015', 'stage-0']
      }
    },
    {
      test: /\.css$/,
      loader: 'style!css'
    },
    {
      test: /\.html$/,
      loader: 'raw'
    }]
  },
  plugins: [
    new WebpackNotifierPlugin(),
    new HtmlWebpackPlugin({
      template: __dirname + '/../src/index.html',
      inject: 'body',
      minify: false
    })
  ]
};
module.exports = config;

I asked on stackoverflow as well: https://stackoverflow.com/questions/35273956/webpack-fails-to-build-es6-in-external-packages