title: Navigator inside PopupMenuItem does not work
body: <p>When trying to call <code>Navigator</code> inside the <code>onTap</code> property of a <code>PopupMenuItem</code> it doesn't work:</p>
  <pre><code>PopupMenuButton(itemBuilder: (BuildContext context) {
    return [
      PopupMenuItem(
        child: Text('Edit'),
        onTap: () =&gt; Navigator.of(context)
            .push(MaterialPageRoute(builder: (context) =&gt; EditorPage())),
      ),
    ];
  }),
  </code></pre>