Make dart2js unittests async.

tests/compiler/dart2js/boolify_test.dart

Index: tests/compiler/dart2js/boolify_test.dart
diff --git a/tests/compiler/dart2js/boolify_test.dart b/tests/compiler/dart2js/boolify_test.dart
index d0293790f6ed62d5e5331dcf330b05b9bd5ba7e8..6536386278f12eb60a75fa915752b96e91ae3ba2 100644
--- a/tests/compiler/dart2js/boolify_test.dart
+++ b/tests/compiler/dart2js/boolify_test.dart
@@ -3,7 +3,9 @@
 // BSD-style license that can be found in the LICENSE file.
 
 library boolified_operator_test;
-import "package:expect/expect.dart";
+
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String TEST = r"""
@@ -15,6 +17,7 @@ foo() {
 """;
 
 main() {
-  String generated = compile(TEST, entry: 'foo');
-  Expect.isTrue(generated.contains('foo() !== true)'));
+  asyncTest(() => compile(TEST, entry: 'foo', check: (String generated) {
+    Expect.isTrue(generated.contains('foo() !== true)'));
+  }));
 }