title: after upgrading flutter, I'm always get "Unblock-file" not recognized. how to get rid of these
body: <p>I tried to upgrade my flutter from 1.12.13 hotfix 7 stable to 1.12.13.hotfix 8 stable, by running <code>flutter upgrade</code> on my windows 7 64 bit machine</p>
  
  <p>now, everytime I run "flutter run", "flutter pub get", etc it runs so slow and sometimes stuck. and I always get some warnings like 'Unblock-file' is not recognized, and "flutter requires PowerShell 5.0 or newer". It also shows "Checking SDK Version...", "building flutter tool..." and "running flutter upgrade..."</p>