title: Property 'ensure' does not exist on type 'NodeRequire'
body: <p>I'm trying webpack 2 code splitting.</p>
  
  <p>According to this doc: <a href="https://webpack.js.org/guides/code-splitting-require/" rel="noreferrer">https://webpack.js.org/guides/code-splitting-require/</a> </p>
  
  <p>the following code should include some.css into a new chunk named 'something'</p>
  
  <pre><code>require.ensure([], function(require) {
      require('some.css');
  }, 'something');
  </code></pre>
  
  <p>but when I run it, I get this error:</p>
  
  <pre><code>ERROR in ./src/index.ts
  (4,9): error TS2339: Property 'ensure' does not exist on type 'NodeRequire'.
  </code></pre>
  
  <p>Any idea about how to fix it?
  Thanks</p>