Make dart2js unittests async.

tests/compiler/dart2js/identity_test.dart

 // Copyright (c) 2011, 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:expect/expect.dart";
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 class A {}
 bool foo(bar) {
   var x = new A();
   var y = new A();
   return identical(x, y);
 }
 """;
 
 main() {
-  String generated = compile(TEST_ONE, entry: 'foo');
+  asyncTest(() => compile(TEST_ONE, entry: 'foo', check: (String generated) {
 
-  // Check that no boolify code is generated.
-  RegExp regexp = new RegExp("=== true");
-  Iterator matches = regexp.allMatches(generated).iterator;
-  Expect.isFalse(matches.moveNext());
+    // Check that no boolify code is generated.
+    RegExp regexp = new RegExp("=== true");
+    Iterator matches = regexp.allMatches(generated).iterator;
+    Expect.isFalse(matches.moveNext());
 
-  regexp = new RegExp("===");
-  matches = regexp.allMatches(generated).iterator;
-  Expect.isTrue(matches.moveNext());
-  Expect.isFalse(matches.moveNext());
+    regexp = new RegExp("===");
+    matches = regexp.allMatches(generated).iterator;
+    Expect.isTrue(matches.moveNext());
+    Expect.isFalse(matches.moveNext());
+  }));
 }