Reapply "Add a sleep function to dart:io"

pkg/scheduled_test/test/scheduled_test/task_return_value_test.dart

Index: pkg/scheduled_test/test/scheduled_test/task_return_value_test.dart
diff --git a/pkg/scheduled_test/test/scheduled_test/task_return_value_test.dart b/pkg/scheduled_test/test/scheduled_test/task_return_value_test.dart
index 0d3ec1e904d5536b762e9c9d09b89ceae1b84a61..e597167e327570be8839609823f4238ec9518726 100644
--- a/pkg/scheduled_test/test/scheduled_test/task_return_value_test.dart
+++ b/pkg/scheduled_test/test/scheduled_test/task_return_value_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;
@@ -79,4 +79,4 @@ void main() {
       expect(error.error, equals('error'));
     });
   }, passing: ['test 2']);
-}
+}