Reapply "Add a sleep function to dart:io"

pkg/scheduled_test/test/scheduled_test/nested_task_test.dart

Index: pkg/scheduled_test/test/scheduled_test/nested_task_test.dart
diff --git a/pkg/scheduled_test/test/scheduled_test/nested_task_test.dart b/pkg/scheduled_test/test/scheduled_test/nested_task_test.dart
index b5154a6bbfac744de6f1374cf28ab6c7f8c6dbc5..2e7a10b4752006b8baed729e04b3b602e8c30bd6 100644
--- a/pkg/scheduled_test/test/scheduled_test/nested_task_test.dart
+++ b/pkg/scheduled_test/test/scheduled_test/nested_task_test.dart
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:io';
+import 'dart:io' hide sleep;
 
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/src/mock_clock.dart' as mock_clock;
@@ -134,4 +134,4 @@ void main() {
       expect(parentTaskFinishedBeforeOnComplete, isTrue);
     });
   }, passing: ['test 2']);
-}
+}