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

tests/language/vm/null_hashcode_optimized_vm_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 that optimized Object.hashCode works for the null receiver.
-// VMOptions=--optimization_counter_threshold=10
+// VMOptions=--optimization_counter_threshold=10 --no-background_compilation
 
 main() {
   for (int i = 0; i < 20; i++) {
     foo(null);
   }
 }
 
 foo(a) => a.hashCode;