Make dart2js unittests async.

tests/compiler/dart2js/null_type_test.dart

Index: tests/compiler/dart2js/null_type_test.dart
diff --git a/tests/compiler/dart2js/null_type_test.dart b/tests/compiler/dart2js/null_type_test.dart
index a4130275adf813985caa51756715273463a2fe0c..c4f76064cebaa19be42e5f7340476567d3d43cc5 100644
--- a/tests/compiler/dart2js/null_type_test.dart
+++ b/tests/compiler/dart2js/null_type_test.dart
@@ -2,7 +2,8 @@
 // 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:expect/expect.dart";
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
@@ -13,6 +14,7 @@ foo() {
 """;
 
 main() {
-  String generated = compile(TEST_ONE, entry: 'foo');
-  Expect.isFalse(generated.contains('typeof (void 0)'));
+  asyncTest(() => compile(TEST_ONE, entry: 'foo', check: (String generated) {
+    Expect.isFalse(generated.contains('typeof (void 0)'));
+  }));
 }