Reapply "Add a sleep function to dart:io"

pkg/scheduled_test/test/scheduled_test/signal_error_test.dart

Index: pkg/scheduled_test/test/scheduled_test/signal_error_test.dart
diff --git a/pkg/scheduled_test/test/scheduled_test/signal_error_test.dart b/pkg/scheduled_test/test/scheduled_test/signal_error_test.dart
index 07cb576dbad1e78160c7688bc57558abf612781a..ba525151032a1263f5f903bcea0c0f1fbff9ac04 100644
--- a/pkg/scheduled_test/test/scheduled_test/signal_error_test.dart
+++ b/pkg/scheduled_test/test/scheduled_test/signal_error_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;
@@ -45,4 +45,4 @@ void main() {
       currentSchedule.signalError('bad');
     });
   });
-}
+}