Make dart2js unittests async.

tests/compiler/dart2js/type_inference2_test.dart

Index: tests/compiler/dart2js/type_inference2_test.dart
diff --git a/tests/compiler/dart2js/type_inference2_test.dart b/tests/compiler/dart2js/type_inference2_test.dart
index a1613ac73e97274e8cc25220ea73189fc06991fe..5e5f224d27eb595de6afcf4b5df9aaa747c5acd2 100644
--- a/tests/compiler/dart2js/type_inference2_test.dart
+++ b/tests/compiler/dart2js/type_inference2_test.dart
@@ -2,6 +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.
 
+import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
@@ -13,5 +14,5 @@ sum(param0, param1) {
 """;
 
 main() {
-  compileAndMatchFuzzy(TEST_ONE, 'sum', r"\+\+x");
+  asyncTest(() => compileAndMatchFuzzy(TEST_ONE, 'sum', r"\+\+x"));
 }