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

tests/language/vm/optimized_guarded_field_isolates_test.dart

 // 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=100
+// VMOptions=--optimization_counter_threshold=100 --no-background_compilation
 
 // Test field type tracking and field list-length tracking in the presence of
 // multiple isolates.
 
 import "dart:isolate";
 import "dart:async";
 import "package:expect/expect.dart";
 import 'package:async_helper/async_helper.dart';
 
 class A {
@@ skipping 51 matching lines @@
       Expect.throws(() => test_c(value), (e) => e is RangeError);
       receive_port.close();
       asyncEnd();
   });
 }
 
 main() {
   test_field_type();
   test_list_length();
 }