[dartfix] Remove unused dart:async imports

Change-Id: I11ae93270e342b3ed5f3081473cb22fd4b5ef483
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/161467
Auto-Submit: Mike Fairhurst <mfairhurst@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: MichaelRFairhurst

pkg/dartfix/lib/listener/bad_message_listener.dart

@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:analysis_server_client/listener/server_listener.dart';
 
 /// [BadMessageListener] throws an exception if the [Client] receives bad data.

pkg/dartfix/lib/src/driver.dart

@@ -2,7 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
 import 'dart:io' show File, Platform;
 
 import 'package:analysis_server_client/handler/connection_handler.dart';