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

tests/language/await_exceptions_test.dart

Index: tests/language/await_exceptions_test.dart
diff --git a/tests/language/await_exceptions_test.dart b/tests/language/await_exceptions_test.dart
index f1d39a199411fe5ab3001bc0f73b5e606d8a239c..c739024154c16a15b06b1b66d51cea46324802d6 100644
--- a/tests/language/await_exceptions_test.dart
+++ b/tests/language/await_exceptions_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";