Make dart2js unittests async.

tests/compiler/dart2js/is_inference2_test.dart

 // Copyright (c) 2012, 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_IF_BOOL_FIRST_INSTRUCTION = r"""
 negate(x) {
   if (x is bool) return !x;
   return x;
 }
 """;
 
 main() {
-  String generated = compile(TEST_IF_BOOL_FIRST_INSTRUCTION, entry: 'negate');
-  Expect.isTrue(generated.contains("!"));  // We want to see !x.
-  Expect.isFalse(generated.contains("!="));  // And not !== true.
-  Expect.isFalse(generated.contains("true"));
-  Expect.isFalse(generated.contains("false"));
+  asyncTest(() => compile(TEST_IF_BOOL_FIRST_INSTRUCTION, entry: 'negate',
+      check: (String generated) {
+    Expect.isTrue(generated.contains("!"));  // We want to see !x.
+    Expect.isFalse(generated.contains("!="));  // And not !== true.
+    Expect.isFalse(generated.contains("true"));
+    Expect.isFalse(generated.contains("false"));
+  }));
 }