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

tests/standalone/slowpath_safepoints_test.dart

Index: tests/standalone/slowpath_safepoints_test.dart
diff --git a/tests/standalone/slowpath_safepoints_test.dart b/tests/standalone/slowpath_safepoints_test.dart
index 254ad7605b05db2a42e8089e537cb4dedf189dd0..e15723cdd7168199463f417f4c642c33a11b2290 100644
--- a/tests/standalone/slowpath_safepoints_test.dart
+++ b/tests/standalone/slowpath_safepoints_test.dart
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 // Test that safepoints associated with slowpaths don't mark non-existing values
 // alive.
-// VMOptions=--optimization-counter-threshold=5 --no-inline_alloc --gc_at_instance_allocation=_Double
+// VMOptions=--optimization-counter-threshold=5 --no-inline_alloc --gc_at_instance_allocation=_Double --no-background-compilation
 
 class C {
   final next;