Make dart2js unittests async.

tests/compiler/dart2js/inverse_operator_test.dart

 // Copyright (c) 2012, 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.
 
+import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String MAIN = r"""
 int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 main() {
   var x = 1;
   if (inscrutable(x) == 0) {
     main();
     x = 2;
   }
   print(!(1 < x));
 }""";
 
 main() {
   // Make sure we don't introduce a new variable.
-  compileAndMatchFuzzy(MAIN, 'main', "1 >= x");
+  asyncTest(() => compileAndMatchFuzzy(MAIN, 'main', "1 >= x"));
 }