Make dart2js unittests async.

tests/compiler/dart2js/dead_phi_eliminator_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"""
 void foo(bar) {
   var toBeRemoved = 1;
   if (bar) {
     toBeRemoved = 2;
   } else {
     toBeRemoved = 3;
   }
 }
 """;
 
 main() {
-  String generated = compile(TEST_ONE, entry: 'foo');
-  RegExp regexp = new RegExp("toBeRemoved");
-  Expect.isTrue(!regexp.hasMatch(generated));
+  asyncTest(() => compile(TEST_ONE, entry: 'foo', check: (String generated) {
+    RegExp regexp = new RegExp("toBeRemoved");
+    Expect.isTrue(!regexp.hasMatch(generated));
+  }));
 }