Make dart2js unittests async.

tests/compiler/dart2js/constant_namer_test.dart

 // Copyright (c) 2013, 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 'package:expect/expect.dart';
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
   class Token {
     final name;
     final value;
     const Token(this.name, [this.value]);
     use() { print(this); }
   }
   test() {
     const [12,53].use();
     const Token('start').use();
     const Token('end').use();
     const Token('yes', 12).use();
     const Token(true, false).use();
   }
 """;
 
 main() {
   check(generated, text) {
     Expect.isTrue(generated.contains(text), text);
   }
 
-  var generated = compile(TEST_ONE, entry: 'test');
-
-  check(generated, '.List_12_53.');
-  check(generated, '.Token_start_null.');
-  check(generated, '.Token_end_null.');
-  check(generated, '.Token_yes_12.');
-  check(generated, '.Token_true_false.');
+  asyncTest(() => compile(TEST_ONE, entry: 'test').then((String generated) {
+    check(generated, '.List_12_53.');
+    check(generated, '.Token_start_null.');
+    check(generated, '.Token_end_null.');
+    check(generated, '.Token_yes_12.');
+    check(generated, '.Token_true_false.');
+  }));
 }