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

tests/language/await_future_test.dart

Index: tests/language/await_future_test.dart
diff --git a/tests/language/await_future_test.dart b/tests/language/await_future_test.dart
index b0151c57f831a1db4b75c530812582ec4c82a07d..2ec30fe8195cd8991c8b1ae78063a04f82438e6c 100644
--- a/tests/language/await_future_test.dart
+++ b/tests/language/await_future_test.dart
@@ -2,7 +2,7 @@
 // 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';
 import "package:async_helper/async_helper.dart";