title: How to add icon to AppBar in Flutter
body: <p>If I have an AppBar like this:</p>
  
  <p><img src="https://i.stack.imgur.com/qhI7e.png" width="300"></p>
  
  <p>How do I add a clickable icon to it like this?</p>
  
  <p><img src="https://i.stack.imgur.com/TTOMn.png" width="300"></p>