[dartdev] Remove unused dart:async import

Change-Id: I800a2f3f85520149e73a541dac4cd1a7a04f1222
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/161466
Auto-Submit: Mike Fairhurst <mfairhurst@google.com>
Commit-Queue: Jaime Wren <jwren@google.com>
Reviewed-by: Jaime Wren <jwren@google.com>

pkg/dartdev/bin/dartdev.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:isolate';
 
 import 'package:dartdev/dartdev.dart';