Add --no-background-compilation to tests that expect to run code in optimized form

tests/language/vm/string_polymorphic_test.dart

Index: tests/language/vm/string_polymorphic_test.dart
diff --git a/tests/language/vm/string_polymorphic_test.dart b/tests/language/vm/string_polymorphic_test.dart
index e7c64101cff7e1b3fd3bd2623cce0ab3215a8162..72eac28b3a5939c6cfe617f76dd8fa95e6922f91 100644
--- a/tests/language/vm/string_polymorphic_test.dart
+++ b/tests/language/vm/string_polymorphic_test.dart
@@ -2,7 +2,7 @@
 // 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.
 // Tests that the VM does not crash on weird corner cases of class Math.
-// VMOptions=--optimization_counter_threshold=10
+// VMOptions=--optimization_counter_threshold=10 --no-background_compilation
 
 import 'package:expect/expect.dart';