Migrated test block 168 to Dart 2.0.

tests/lib/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;
-  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();
-  });
-}