pkg: analysis aided cleanup

pkg/scheduled_test/test/scheduled_test/nested_task_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 'dart:io' hide sleep;
 
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/src/mock_clock.dart' as mock_clock;
 
 import '../metatest.dart';
 import '../utils.dart';
 
 void main() {
   setUpTimeout();
 
@@ skipping 112 matching lines @@
           parentTaskFinished = true;
         });
       });
     });
 
     test('test 2', () {
       expect(parentTaskFinishedBeforeOnComplete, isTrue);
     });
   }, passing: ['test 2']);
 }