title: How to make a full screen dialog in flutter?
body: <p>I want to make a full screen dialog box. Dialog box background must be opaque. 
  Here is an example:
  <a href="https://i.stack.imgur.com/U5W5i.png" rel="noreferrer"><img src="https://i.stack.imgur.com/U5W5i.png" alt="enter image description here"></a></p>
  
  <p>How to make like this in Flutter?</p>