pkg/scheduled_test: removed unused imports

pkg/scheduled_test/test/descriptor/async_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // 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:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../metatest.dart';
 import 'utils.dart';
 
 void main(_, message) {
   initMetatest(message);
 
   setUpTimeout();
@@ skipping 96 matching lines @@
       })).validate();
     });
 
     test('test 2', () {
       expect(errors.single, new isInstanceOf<ScheduleError>());
       expect(errors.single.error.toString(),
           matches(r"^Directory not found: '[^']+[\\/]dir'\.$"));
     });
   }, passing: ['test 2']);
 }