title: Why Material-UI is not recognizing the theme.spacing function?
body: <h1>Description</h1>
  <p>I am trying to use Material-UI's <strong>theme.spacing</strong> function in a React application but the <strong>spacing</strong> function is not recognized.</p>
  <p>The Javascript error message is: <strong>TypeError: theme.spacing is not a function</strong></p>
  <p>I am unsure if this is a bug or I have something wrong with the frameworks versions being installed.</p>
  <hr />
  <h1>Context</h1>
  <ul>
  <li>Here's <a href="https://github.com/mui-org/material-ui/issues/14474" rel="noreferrer">the Github issue</a> raised to address this problem.</li>
  <li>Here's <a href="https://github.com/lesair/create-react-app" rel="noreferrer">the repo</a> with the offending code. The error is caught in the <a href="https://github.com/mui-org/material-ui/blob/master/examples/create-react-app/src/pages/index.js" rel="noreferrer">src\pages\index.js</a> file, line 16:
  <code>paddingTop: theme.spacing(20)</code></li>
  <li>Here's <a href="https://codesandbox.io/s/github/lesair/create-react-app" rel="noreferrer">the sandboxed repo running</a> where you can actually see the error message.</li>
  </ul>
  <p>These are the frameworks versions being installed, according to the <strong>package-lock.json</strong> file:</p>
  <ul>
  <li>Material-UI: v3.9.2</li>
  <li>React: v16.8.1</li>
  <li>Chrome: v72.0.3626.96</li>
  <li>TypeScript: None</li>
  <li>create-react-app: v3.2.2</li>
  </ul>