title: Mac OS Big sur Flutter install error, “Bad state: Future already completed”
body: <p>it's showing like this.</p>
  <pre><code>Unhandled exception:
  Bad state: Future already completed
  #0      _AsyncCompleter.complete (dart:async/future_impl.dart:43:31)
  #1      _NativeSocket.startConnect.&lt;anonymous closure&gt;.connectNext.&lt;anonymous closure&gt; (dart:io-patch/socket_patch.dart:678:23)
  #2      _NativeSocket.issueWriteEvent.issue (dart:io-patch/socket_patch.dart:1043:14)
  #3      _NativeSocket.issueWriteEvent (dart:io-patch/socket_patch.dart:1050:12)
  #4      _NativeSocket.multiplex (dart:io-patch/socket_patch.dart:1071:11)
  #5      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)
  Error: Unable to 'pub upgrade' flutter tool. Retrying in five seconds...
  </code></pre>
  <p>I totally uninstall flutter from macOS Big Sur. but it's still showing the same result.</p>