title: Flutter cannot delete build folder when flutter clean is executed
body: <p>Whenever I try to clean my flutter project by using the command 
  <code>flutter clean</code> I get an error saying </p>
  
  <pre><code>Failed to remove build. A program may still be using a file in the directory or the directory itself. To find and stop such a program, see:
  https://superuser.com/questions/1333118/cant-delete-empty-folder-because-it-is-used
  </code></pre>
  
  <p>I can understand that there is some directory opened and it has to closes in order to clean. But I cannot understand which one is it. I tried <strong>restarting my computer</strong> to resolve this issue. But still when I run the command <strong>flutter clean</strong> I get the error. </p>