title: Getting undefined components in React Router v4
body: <p>Just upgraded to <code>react-router-dom 4.0.0</code>. All my components are either regular <code>class</code>es or fat arrows. They are all exported using <code>export default ThatComponent</code>. Yet I'm getting this:</p>
  
  <p><em>Uncaught Error: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) <strong>but got: undefined.</strong> You likely forgot to export your component from the file it's defined in. Check the render method of <code>Router</code>.</em></p>
  
  <pre><code>// minimal showcase
  import { BrowserRouter, Match, Miss } from 'react-router';
  
  const Router = () =&gt; (
    &lt;BrowserRouter&gt;
      &lt;div&gt;
        {/* both Match and Miss components below cause an error */}
        &lt;Match exactly pattern="/login" component={Login} /&gt; 
        &lt;Match exactly pattern="/frontpage" component={Frontpage} /&gt;
        &lt;Match exactly pattern="/logout" render={() =&gt; (&lt;div&gt;logout&lt;/div&gt;)} /&gt;
        &lt;Miss component={NoMatch} /&gt;
      &lt;/div&gt;
    &lt;/BrowserRouter&gt;
  );
  </code></pre>
  
  <p>Why do the <code>&lt;Match&gt;</code> components think the other components are undefined?</p>