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

tests/language/vm/optimized_guarded_field_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.
 // Test correct handling of phis with only environment uses that were inserted
 // by store to load forwarding.
-// VMOptions=--optimization_counter_threshold=10
+// VMOptions=--optimization_counter_threshold=10 --no-background_compilation
 
 import "package:expect/expect.dart";
 import "dart:typed_data";
 
 class A {
   var foo;
 }
 
 class B {
   get foo => null;
@@ skipping 75 matching lines @@
   for (var i = 0; i < 20; i++) { test_deopt(42, 1); }
   Expect.equals(43, test_deopt(42, 1));
   Expect.equals("aaabbb", test_deopt("aaa", "bbb"));
 
   // Regression test for fields initialized in native code (Error._stackTrace).
   test_stacktrace();
 
   // Test guarded list length.
   for (var i = 0; i < 20; i++) test_guarded_length();
 }