title: ERROR in multi (webpack)-dev-server/client?http://localhost:8080 ./src when use webpack4.2.0
body: <p>when i use webpack4.2.0, play 'run start', show errors follow:</p>
  
  <p>WARNING in configuration
  The 'mode' option has not been set. Set 'mode' option to 'development' or 'production' to enable defaults for this environment.</p>
  
  <p>ERROR in multi (webpack)-dev-server/client?<a href="http://localhost:8001" rel="noreferrer">http://localhost:8001</a> ./src
  Module not found: Error: Can't resolve './src' in '/Users/xudengwei/projects/xudengwei/myOpenGithub/angular5-scaffold'
   @ multi (webpack)-dev-server/client?<a href="http://localhost:8001" rel="noreferrer">http://localhost:8001</a> ./src
  ℹ ｢wdm｣: Failed to compile.</p>
  
  #
  
  <p>my  configure as follow:
  webpack.common.js:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>var webpack = require('webpack');
  var HtmlWebpackPlugin = require('html-webpack-plugin');
  
  module.exports = {
      entry: {
          'polyfills': './src/polyfills.ts',
          'vendor': './src/vendor.ts',
          'app': './src/app/main.ts'
      },
      resolve: {
          extensions: ['.ts', '.js']
      },
      module: {
          rules: [
              {
                  test: /\.tsx?$/,
                  loader: "ts-loader"
              }
          ]
      },
      plugins: [
          new HtmlWebpackPlugin({
              template: './index.html',
              inject: true
          })
      ]
  };</code></pre>
  </div>
  </div>
  </p>
  
  <p>webpack.dev.js:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>var path = require('path');
  var webpackMerge = require('webpack-merge');
  var commonConfig = require('./webpack.common.js');
  
  module.exports = webpackMerge(commonConfig, {
      output: {
          path: path.resolve(__dirname, 'dist'),
          filename: '[name].js',
          chunkFilename: '[id].chunk.js'
      },
      module: {
          rules: []
      },
      plugins: [    
          new HtmlWebpackPlugin({
              template: './index.html',
              inject: true
          })
      ],
      devServer: {
          devServer: {
              contentBase: './dist'
          },
      }
  });</code></pre>
  </div>
  </div>
  </p>
  
  <p>someone knows the reason?thanks</p>