Make dart2js unittests async.

tests/compiler/dart2js/strength_eq_test.dart

 // Copyright (c) 2014, 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.
 // Test constant folding on numbers.
 
-import "package:expect/expect.dart";
+import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String CODE = """
 class A {
   var link;
 }
 int foo(x) {
   if (new DateTime.now().millisecondsSinceEpoch == 42) return null;
   var a = new A();
   if (new DateTime.now().millisecondsSinceEpoch == 42) return a;
   a.link = a;
   return a;
 }
 void main() {
   var x = foo(0);
   return x == x.link;
 }
 """;
 
 main() {
   // The `==` is strengthened to a HIdentity instruction.  The HIdentity follows
   // `x.link`, so x cannot be `null`.
   var compare = new RegExp(r'x === x\.get\$link\(\)');
-  compileAndMatch(CODE, 'main', compare);
+  asyncTest(() => compileAndMatch(CODE, 'main', compare));
 }