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

tests/language/constant_propagation_phis_test.dart

Index: tests/language/constant_propagation_phis_test.dart
diff --git a/tests/language/constant_propagation_phis_test.dart b/tests/language/constant_propagation_phis_test.dart
index e7e2d809a479ed1c66ef5e40dc83bebefb1f3902..0ba7d725382d2d40739273595c51a96921863fd9 100644
--- a/tests/language/constant_propagation_phis_test.dart
+++ b/tests/language/constant_propagation_phis_test.dart
@@ -1,7 +1,7 @@
 // Copyright (c) 2013, 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.
-// VMOptions=--optimization-counter-threshold=10
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation
 
 import "package:expect/expect.dart";