Update test's options to optimize deterministically (turn off background compilation)

tests/lib/convert/json_test.dart

Index: tests/lib/convert/json_test.dart
diff --git a/tests/lib/convert/json_test.dart b/tests/lib/convert/json_test.dart
index cfed8d9e0429edf0824cc47985043b3a82f63ab4..ef7b077d4c8c622a5f59044de9fd3b9c8073236f 100644
--- a/tests/lib/convert/json_test.dart
+++ b/tests/lib/convert/json_test.dart
@@ -1,6 +1,8 @@
 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
+// Disable background compilation so that the Issue 24908 can be reproduced.
+// VMOptions=--no-background-compilation
 
 library json_test;