title: Add a divider on top of a bottom navigation bar?
body: <p>Is there a way to add a divider like this one to the top of a <code>BottomNavigationBar()</code>? I got it for the <code>AppBar()</code> as you can see below:</p>
  <p><img src="https://i.stack.imgur.com/q4nlU.png" alt="AppBar() with Divider()" /></p>
  <pre><code>appBar: AppBar(
          elevation: 0,
          title: Text(
            _views[_index]['title'],
          ),
          bottom: PreferredSize(
            preferredSize: Size.fromHeight(10),
            child: Divider(
              thickness: 2,
              height: 0,
            ),
          ),
        ),
  </code></pre>
  <p>But I wonder if there is a way to do the same with the <code>BottomNavigationBar()</code>.</p>
  <p>It should look like this in the end:</p>
  <p><img src="https://i.stack.imgur.com/YeVeD.png" alt="BottomNavigationBar()" /></p>