Make dart2js unittests async.

tests/compiler/dart2js/builtin_interceptor_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 'dart:async';
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 foo() {
   return "foo".length;
 }
 """;
 
 const String TEST_TWO = r"""
 foo() {
   return r"foo".length;
 }
 """;
 
 const String TEST_THREE = r"""
 foo() {
   return new List().add(2);
 }
 """;
 
 main() {
-  String generated = compile(TEST_ONE, entry: 'foo');
-  Expect.isTrue(generated.contains("return 3;"));
-
-  generated = compile(TEST_TWO, entry: 'foo');
-  Expect.isTrue(generated.contains("return 3;"));
-
-  generated = compile(TEST_THREE, entry: 'foo');
-  Expect.isTrue(generated.contains("push(2);"));
+  asyncTest(() => Future.wait([
+    compile(TEST_ONE, entry: 'foo', check: (String generated) {
+      Expect.isTrue(generated.contains("return 3;"));
+    }),
+    compile(TEST_TWO, entry: 'foo', check: (String generated) {
+      Expect.isTrue(generated.contains("return 3;"));
+    }),
+    compile(TEST_THREE, entry: 'foo', check: (String generated) {
+      Expect.isTrue(generated.contains("push(2);"));
+    }),
+  ]));
 }