Use Directory.systemTemp getter in pkg subdirectory of repository.

pkg/scheduled_test/test/descriptor/utils.dart

Index: pkg/scheduled_test/test/descriptor/utils.dart
diff --git a/pkg/scheduled_test/test/descriptor/utils.dart b/pkg/scheduled_test/test/descriptor/utils.dart
index bd9ae57ed3f4eee23db4033b9d41650106e0e55a..9674469dbc5d269c10b58c847b5062eea46f010a 100644
--- a/pkg/scheduled_test/test/descriptor/utils.dart
+++ b/pkg/scheduled_test/test/descriptor/utils.dart
@@ -16,7 +16,7 @@ String sandbox;
 
 void scheduleSandbox() {
   schedule(() {
-    return new Directory('').createTemp().then((dir) {
+    return Directory.systemTemp.createTemp('descriptor_sandbox_').then((dir) {
       sandbox = dir.path;
       d.defaultRoot = sandbox;
     });