title: Webpack 2 extract text plugin entrypoints.length issues
body: <p>I have an issue with upgrading to webpack 2 and the extract text plugin. I have the dev version (without this plugin) working and I cant see whats different. The error I get is</p>
  
  <pre><code>node_modules\webpack\lib\Chunk.js:62
              return this.entrypoints.length &gt; 0;
  
  TypeError: Cannot read property 'length' of undefined
  </code></pre>
  
  <p>I have gulp running webpack 2 and this plugin for a single css file. This is the main part of my webpack config (happy to provide it all if needed):</p>
  
  <pre><code>     ...
       modules: { rules :[ ... {
           test: /\.scss/,
           exclude: /node_modules/,
           use: [
               "style-loader?sourceMap",
               {
                   loader: "css-loader",
                   options: {
                       minimize: true,
                       modules: true,
                       importLoaders: true,
                       localIdentName: "[path]___[name]__[local]___[hash:base64:5]",
                   },
               },
               {
                   loader: "postcss-loader",
                   options: { ...postCSSConfig },
               },
               {
                   loader: "sass-loader",
                   options: { includePaths: [path.join(process.cwd(), "src", "Styles", "Includes")] },
               },
           ],
       }, ]},
       plugins: [
           new ExtractTextPlugin({
               filename: "[contenthash].css",
               allChunks: true,
           }), ...
  </code></pre>
  
  <p>Im using the following versions:</p>
  
  <p>Webpack: 2.3.3</p>
  
  <p>Extract text plugn: 2.0.1</p>
  
  <p><strong>Edit</strong>: Here is my entry point,</p>
  
  <pre><code>context: path.resolve(process.cwd(), "./src/"),
  entry: [
      "babel-polyfill",
      "whatwg-fetch",
      "Boot",
  ],
  devtool: "eval",
  resolve: {
      modules: ["src", "node_modules"],
      extensions: [".js", ".jsx"],
  },
  </code></pre>
  
  <p>Any ideas would be great. 
  Thanks in advance.</p>