title: Inner shadow effect in flutter
body: <p>According to the <a href="https://github.com/flutter/flutter/issues/18636" rel="noreferrer">github issue</a> there is no inset attribute in ShadowBox yet. Is there any workaround how to emulate inner shadow right now in flutter.</p>
  
  <p>I like to achieve inner shadow effects like you can see on the following images</p>
  
  <p><a href="https://i.stack.imgur.com/7q5Th.png" rel="noreferrer"><img src="https://i.stack.imgur.com/7q5Th.png" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/5UeUH.png" rel="noreferrer"><img src="https://i.stack.imgur.com/5UeUH.png" alt="enter image description here"></a></p>