Migrate async tests to strong

tests/lib_strong/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();
+  });
+}