title: webpackJsonp is not defined: webpack-dev-server and CommonsChunkPlugin
body: <p>This is my webpack.config.js file:</p>
  
  <pre><code>const webpack = require('webpack');
  const path = require('path');
  
  module.exports = {
    cache: true,
    devtool: 'source-map',
    entry: {
      app : [
        './src/index.js'
      ],
      vendor: ['lodash']
    },
    output: {
      filename: 'bundle.js',
      path: path.join(__dirname, 'dist'),
      publicPath: '/dist/',
      pathinfo: true
    },
    module: {
      loaders: [
        { test: /\.js$/, exclude: /node_modules/, loaders: ['babel'] },
        { test: /\.scss/, exclude: /node_modules/, loaders: ['style', 'css', 'sass'] }
      ]
    },
    plugins: [
      new webpack.NoErrorsPlugin(),
      new webpack.optimize.CommonsChunkPlugin('vendor', 'vendor.bundle.js', Infinity)
    ]
  };
  </code></pre>
  
  <p>And this is my scripts that runs the webpack-dev-server:</p>
  
  <pre><code>const webpack =require('webpack');
  const WebpackDevServer = require('webpack-dev-server');
  const webpackConfig = require('../webpack.config');
  const _  = require('lodash');
  
  const webpackDevConfig = _.cloneDeep(webpackConfig);
  const devPort = 3000;
  
  webpackDevConfig.entry.app.unshift('webpack/hot/dev-server');
  webpackDevConfig.entry.app.unshift('webpack-dev-server/client?http://localhost:' + devPort + '/');
  webpackDevConfig.plugins.push(new webpack.HotModuleReplacementPlugin());
  
  new WebpackDevServer(webpack(webpackDevConfig), {
    publicPath: webpackConfig.output.publicPath,
    hot: true,
    stats: {
      colors: true,
      chunks: false
    }
  }).listen(devPort, 'localhost');
  </code></pre>
  
  <p>The <code>webpack</code> command output is good (bundle.js and vendor.bundle.js), however, the dev server fails with <code>webpackJsonp is not defined</code> (although its in-memory compilation succeeded). </p>
  
  <p>When removing <code>CommonsChunkPlugin</code> from webpack.config.js - it all works fine:</p>
  
  <pre><code>...
  entry: [
      './src/index.js'
    ],
  ...
  plugins: [
      new webpack.NoErrorsPlugin()
    ]
  </code></pre>
  
  <p>Any ideas?</p>