Make dart2js unittests async.

tests/compiler/dart2js/dead_phi_eliminator_test.dart

Index: tests/compiler/dart2js/dead_phi_eliminator_test.dart
diff --git a/tests/compiler/dart2js/dead_phi_eliminator_test.dart b/tests/compiler/dart2js/dead_phi_eliminator_test.dart
index 4551e3406aeb0abd4b42b66aec4f6cfb9bdcf94e..a7088d50ffc68958073f09ab8c8e779d181f1ab7 100644
--- a/tests/compiler/dart2js/dead_phi_eliminator_test.dart
+++ b/tests/compiler/dart2js/dead_phi_eliminator_test.dart
@@ -2,7 +2,8 @@
 // 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"""
@@ -17,7 +18,8 @@ void foo(bar) {
 """;
 
 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));
+  }));
 }