title: Flutter make simple gradient shadow
body: <p>I want to build a gradient shadow widget as shown below. </p>
  
  <p><a href="https://i.stack.imgur.com/m4Skt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/m4Skt.png" alt="enter image description here"></a></p>
  
  <p>This gradient starts from black and end at white, how can I design this type of widget?</p>