Reapply "Add a sleep function to dart:io"

pkg/scheduled_test/test/scheduled_test/out_of_band_task_test.dart

Index: pkg/scheduled_test/test/scheduled_test/out_of_band_task_test.dart
diff --git a/pkg/scheduled_test/test/scheduled_test/out_of_band_task_test.dart b/pkg/scheduled_test/test/scheduled_test/out_of_band_task_test.dart
index 34f35f0e966a356e514b95b9e8d42293a0cab80a..35bd68e66b7ea652e96e2c2d61dca8d9d30554b1 100644
--- a/pkg/scheduled_test/test/scheduled_test/out_of_band_task_test.dart
+++ b/pkg/scheduled_test/test/scheduled_test/out_of_band_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;
@@ -46,4 +46,4 @@ void main() {
       currentSchedule.onComplete.schedule(() => expect(scheduleRun, isTrue));
     });
   });
-}
+}