Reapply "Add a sleep function to dart:io"

pkg/scheduled_test/test/scheduled_test/current_schedule_errors_test.dart

Index: pkg/scheduled_test/test/scheduled_test/current_schedule_errors_test.dart
diff --git a/pkg/scheduled_test/test/scheduled_test/current_schedule_errors_test.dart b/pkg/scheduled_test/test/scheduled_test/current_schedule_errors_test.dart
index 715ac52d52fdaea2fb314c1180675e136636f14c..4766b431f2d66a525c656ec5a45551172dfa3d12 100644
--- a/pkg/scheduled_test/test/scheduled_test/current_schedule_errors_test.dart
+++ b/pkg/scheduled_test/test/scheduled_test/current_schedule_errors_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;
@@ -236,4 +236,4 @@ void main() {
       expect(errors.map((e) => e.error), equals(['in-band', 'out-of-band']));
     });
   }, passing: ['test 2']);
-}
+}