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

tests/language/nullaware_opt_test.dart

 // Copyright (c) 2015, 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=5
+// VMOptions=--optimization_counter_threshold=5 --no-background_compilation
 //
 // Basic null-aware operator test that invokes the optimizing compiler.
 
 import "package:expect/expect.dart";
 
 class C {
   C(this.f);
   var f;
   m(a) => a;
 }
@@ skipping 51 matching lines @@
   c?.v;
   c?.m(bomb());
 }
 
 main() {
   for (int i = 0; i < 10; i++) {
     test();
     test2();
   }
 }