title: workbox not picking up in-memory files with webpack-dev-server
body: <p>In our project we are using webpack-dev-server to run our development environment.
  We are currently transforming the project into a progressive-web-app using <a href="https://developers.google.com/web/tools/workbox/" rel="noreferrer">workbox</a>.</p>
  
  <p>When we run webpack with NODE_ENV=production everything is working fine, as it first compiles the files, and then the workbox plugin picks them up and adds them to the service-worker.</p>
  
  <p>When we run webpack-dev-server hot, the build fails when running the workbox plugin as it cannot find any files in the dist folder.</p>
  
  <p>The problem seems to be that workbox is not picking up the js and css files that are generated in-memory and seems to need the files on the file system.</p>
  
  <pre><code>module.exports = {
    context: path.resolve(__dirname, 'front'),
    entry: [
      ...preEntries,
      './react/app.js'
    ],
    output: {
      path: path.resolve(__dirname, 'front-dist'),
      filename: `react/app.${git.gitCommitAbbrev}.js`,
      chunkFilename: `react/[id].app.${git.gitCommitAbbrev}.js`,
      publicPath: '/glass/'
    },
    devtool: isProdEnv ? false : 'eval-source-map',
    stats: {
      chunkModules: false
    },
    module: {
      ...
    },
    plugins: [
      new WorkboxPlugin({
        globDirectory: path.resolve(__dirname, 'front-dist'),
        globPatterns: ['**/*.{html,js,css,woff2}'],
        swDest: path.join(path.resolve(__dirname, 'front-dist/sw/'), 'service-worker.js'),
        handleFetch: true,
        clientsClaim: true,
        skipWaiting: true,
      })
    ]
  }
  </code></pre>
  
  <p>Any idea's as to how this should be working</p>