Make dart2js unittests async.

tests/compiler/dart2js/code_motion_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"""
 foo(int a, int b, bool param2) {
   // Make sure a and b become numbers.
   var c = a - b;
   for (int i = 0; i < 1; i++) {
     if (param2) {
       print(a + b);
     } else {
       print(a + b);
     }
   }
   return c;
 }
 """;
 
 main() {
-  String generated = compile(TEST_ONE, entry: 'foo');
-  RegExp regexp = new RegExp('a \\+ b');
-  Iterator matches = regexp.allMatches(generated).iterator;
-  Expect.isTrue(matches.moveNext());
-  Expect.isFalse(matches.moveNext());
+  asyncTest(() => compile(TEST_ONE, entry: 'foo', check: (String generated) {
+    RegExp regexp = new RegExp('a \\+ b');
+    Iterator matches = regexp.allMatches(generated).iterator;
+    Expect.isTrue(matches.moveNext());
+    Expect.isFalse(matches.moveNext());
+  }));
 }