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

tests/language/assign_op_test.dart

Index: tests/language/assign_op_test.dart
diff --git a/tests/language/assign_op_test.dart b/tests/language/assign_op_test.dart
index 9a9ebe7086ddfaf73bcdbfd4df408aaa05262637..a531f319033bc108f214b1bc2e1730c8d34aea2b 100644
--- a/tests/language/assign_op_test.dart
+++ b/tests/language/assign_op_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.
 // Dart test program for testing assign operators.
-// VMOptions=--optimization-counter-threshold=10
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation
 
 import "package:expect/expect.dart";