title: Cannot minify CSS file
body: <p>I'm trying to use <strong>Webpack 4</strong> for my project. All plugins works except <code>extract-text-webpack-plugin</code>.</p>
  
  <p><strong>Actual Behavior</strong>: when I build the project there are no errors at all and minified file also</p>
  
  <p><strong>Expected behavior</strong>: get minified CSS file ( <code>styles.css</code> ) in <code>dist</code> folder</p>
  
  <p><strong>output</strong></p>
  
  <p><a href="https://i.stack.imgur.com/PYhxu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/PYhxu.png" alt="terminal"></a></p>
  
  <p><strong>file structure</strong></p>
  
  <p><a href="https://i.stack.imgur.com/THOSJ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/THOSJ.png" alt="files"></a></p>
  
  <p><strong>webpack.config</strong></p>
  
  <pre><code>const UglifyJsPlugin = require('uglifyjs-webpack-plugin');
  const HtmlWebpackPlugin = require('html-webpack-plugin');
  const ExtractTextPlugin = require('extract-text-webpack-plugin');
  
  module.exports = {
      entry: {
          'index': './src/index.js',
      },
      resolveLoader: {
          modules: [
              'node_modules',
          ],
      },
      mode: 'production',
      module: {
          rules: [
              {
                  test: /\.html$/,
                  use: [
                      {
                          loader: 'html-loader',
                          options: {
                              minimize: true,
                          },
                      },
                  ],
              },
              {
                  test: /\.css$/,
                  use: ExtractTextPlugin.extract( 'css-loader' ),
              },
              {
                  test: /\.(png|jpg|gif)$/,
                  use: [
                      {
                          loader: 'file-loader',
                          options: {
                              name: '[path][name].[ext]',
                              emitFile: false,
                          },
                      },
                  ],
              },
          ],
      },
      plugins: [
          new ExtractTextPlugin( {
              filename: './src/styles.css',
          }),
          new HtmlWebpackPlugin({
              template: './src/index.html',
              filename: 'index.html',
              inject: 'body',
              hash: true,
              minify: {
                  removeAttributeQuotes: true,
                  collapseWhitespace: true,
                  html5: true,
                  removeComments: true,
                  removeEmptyAttributes: true,
                  minifyCSS: true,
              },
          }),
          new UglifyJsPlugin({
              cache: true,
              parallel: true,
              uglifyOptions: {
                  compress: false,
                  ecma: 6,
                  mangle: true,
              },
              sourceMap: true,
          }),
      ],
  };
  </code></pre>