title: Webpack Unrecognized Input because of third party less file using \9
body: <p>So, I am using Ant Design systems Menu component ( I am using other components from them, and all is good ), but this Menu component uses some css that has a IE9 fallback, "\9".. and its killing my build process.</p>
  
  <p>error:</p>
  
  <pre><code>ERROR in ./node_modules/css-loader!./node_modules/postcss-loader/src??ref--3-3!./node_modules/less-loader/dist/cjs.js!./node_modules/antd/lib/menu/style/index.css
  Module build failed: 
  
    background: #fff;
    background: rgba(0, 0, 0, 0.65) \9;
                                  ^
  Unrecognised input
        in /Users/dd/BRANCHES/sandbox/code/ui/node_modules/antd/lib/menu/style/index.css (line 244, column 34)
  </code></pre>
  
  <p>I am guessing this is a webpack issue, and this what I have:</p>
  
  <pre><code>{
          test: /\.(less|css)$/,
          use: ExtractTextPlugin.extract({
            fallback: 'style-loader',
            use: [
              { loader: 'css-loader' },
              {
                loader: 'postcss-loader',
                options: {
                  plugins: function() {
                    return [autoprefixer];
                  }
                }
              },
              { loader: 'less-loader' }
            ]
          })
  </code></pre>
  
  <p>I am using: "webpack": "^3.7.1", (( can't upgrade at this time )).</p>
  
  <p>Any ideas on how to fix?</p>