Make dart2js unittests async.

tests/compiler/dart2js/is_inference2_test.dart

Index: tests/compiler/dart2js/is_inference2_test.dart
diff --git a/tests/compiler/dart2js/is_inference2_test.dart b/tests/compiler/dart2js/is_inference2_test.dart
index 2ce07d5ff3c5c010da170369f473c455a36c4bd3..3e3e6ea436f23c6d7bc1a618d83caa68109a69d5 100644
--- a/tests/compiler/dart2js/is_inference2_test.dart
+++ b/tests/compiler/dart2js/is_inference2_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_IF_BOOL_FIRST_INSTRUCTION = r"""
@@ -13,9 +14,11 @@ negate(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"));
+  }));
 }