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

tests/language/await_nonfuture_test.dart

 // Copyright (c) 2014, 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
 
 import 'package:expect/expect.dart';
 
 var X = 0;
 
 foo() async {
   Expect.equals(X, 10);  // foo runs after main returns.
   return await 5;
 }
 
 main() {
   var f = foo();
   f.then((res) => print("f completed with $res"));
   X = 10;
 }