Migrated test block 168 to Dart 2.0.

tests/lib_2/async/future_microtask_test.dart

 // Copyright (c) 2013, 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:async_helper/async_helper.dart';
 
 main() {
   asyncStart();
-  var x = 499;
+  dynamic x = 499;
   scheduleMicrotask(() {
     Expect.equals(499, x);
     x = 42;
   });
   new Future.microtask(() {
     Expect.equals(42, x);
     x = null;
     return 99;
   }).then((val) {
     Expect.isNull(x);
     Expect.equals(99, val);
     x = "foo";
   });
   scheduleMicrotask(() {
     Expect.equals("foo", x);
     x = "toto";
     asyncEnd();
   });
 
   asyncStart();
   new Future.microtask(() {
     throw "foo";
   }).catchError((e, stackTrace) {
     Expect.equals("foo", e);
     asyncEnd();
   });
 }