Add an async descriptor to scheduled_test.

pkg/scheduled_test/lib/src/descriptor/async.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.
+
+library descriptor.async;
+
+import 'dart:async';
+import 'dart:io' as io;
+
+import '../../descriptor.dart' as descriptor;
+import '../../scheduled_test.dart';
+import '../utils.dart';
+import 'utils.dart';
+
+/// A descriptor that wraps a [Future<Entry>] and forwards all asynchronous
+/// operations to the result of the future. It's designed for use when the
+/// full filesystem description isn't known when initializing the schedule.
+///
+/// Async descriptors don't support [load], since their names aren't
+/// synchronously available.
+class Async extends descriptor.Entry {
+  /// The [Future] that will complete to the [Entry] this descriptor is
+  /// wrapping.
+  final Future<descriptor.Entry> future;
+
+  Async(this.future)
+      : super('<async descriptor>');
+
+  Future create([String parent]) =>
+    schedule(() => future.then((entry) => entry.create(parent)));
+
+  Future validate([String parent]) =>
+    schedule(() => future.then((entry) => entry.validate(parent)));
+
+  Stream<List<int>> load(String path) => errorStream("Async descriptors don't "
+      "support load().");
+
+  Stream<List<int>> read() => errorStream("Async descriptors don't support "
+      "read().");
+
+  String describe() => "async descriptor";
+}