Make dart2js unittests async.

tests/compiler/dart2js/switch_empty_default_test.dart

 // Copyright (c) 2014, 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.
 // Test constant folding on numbers.
 
-import "package:expect/expect.dart";
+import 'dart:async';
+import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String SIMPLY_EMPTY = """
 int foo(x) => x;
 void main() {
   var a = foo(4);
   var b = 1;
   switch (a) {
     case 1:
       b += 2;
@@ skipping 95 matching lines @@
   }
   print(a+b);
 }
 """;
 
 main() {
   var def = new RegExp(r"default:");
   var defOrCase3 = new RegExp(r"(default:|case 3):");
   var case3 = new RegExp(r"case 3:");
 
-  compileAndDoNotMatch(SIMPLY_EMPTY, 'main', def);
-  compileAndDoNotMatch(TOTAL, 'main', defOrCase3);
-  compileAndDoNotMatch(OPTIMIZED, 'main', def);
-  compileAndMatch(LABEL, 'main', case3);
-  compileAndMatch(DEFLABEL, 'main', def);
-  compileAndMatch(EMPTYDEFLABEL, 'main', def);
+  asyncTest(() => Future.wait([
+    compileAndDoNotMatch(SIMPLY_EMPTY, 'main', def),
+    compileAndDoNotMatch(TOTAL, 'main', defOrCase3),
+    compileAndDoNotMatch(OPTIMIZED, 'main', def),
+    compileAndMatch(LABEL, 'main', case3),
+    compileAndMatch(DEFLABEL, 'main', def),
+    compileAndMatch(EMPTYDEFLABEL, 'main', def),
+  ]));
 }