title: Webpack / PostCSS - OptimiseCssAssetsPlugin stopping CSS Sourcemap from being available in browser
body: <p>I am trying to use OptimiseCssAssetsPlugin within webpack, however when I introduce it, my css sourcemap, which appears to still be generated, is no longer available within chrome.  </p>
  
  <p>My webpack.config.js is the following</p>
  
  <pre><code>const webpack = require('webpack');
  const path = require('path');
  var precss       = require('precss');
  var autoprefixer = require('autoprefixer');
  var ExtractTextPlugin = require("extract-text-webpack-plugin");
  var postcssImport = require('postcss-import');
  var OptimizeCssAssetsPlugin = require('optimize-css-assets-webpack-plugin');
  
  module.exports = {
  
    context: __dirname + '/frontend',
    devtool: 'source-map',
    entry: "./index.js",
    output: {
      filename: 'bundle.js',
      path: path.join(__dirname, './static')
  },
    module: {
      loaders: [
        {
          test: /\.js$/,
          loader: 'babel',
          exclude: /node_modules/,
          query: {
            presets: ['es2015']
          }
        },
        {
          test:   /\.css$/,
          loader: ExtractTextPlugin.extract('style-loader', '!css-loader?sourceMap&amp;importLoaders=1!postcss-loader')
        }
      ]
    },
    plugins: [
      new ExtractTextPlugin("si-styles.css"),
      new OptimizeCssAssetsPlugin()
    ],
     postcss: function(webpack) {
          return [
              postcssImport({ addDependencyTo: webpack }), // Must be first item in list
              precss,
              autoprefixer({ browsers: ['last 2 versions']  })
          ];
      },
  
  }
  </code></pre>
  
  <p>Any thoughts on this would be greatly appreciated.</p>