Add a scheduled test library.

pkg/scheduled_test/lib/src/task.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 task;
+
+import 'dart:async';
+
+import 'schedule.dart';
+import 'utils.dart';
+
+typedef Future TaskBody();
+
+/// A single task to be run as part of a [TaskQueue].
+class Task {
+  /// The queue to which this [Task] belongs.
+  final TaskQueue queue;
+
+  /// A description of this task. Used for debugging. May be `null`.
+  final String description;
+
+  /// The body of the task.
+  TaskBody fn;
+
+  /// The identifier of the task. This is unique within [queue]. It's used for
+  /// debugging when [description] isn't provided.
+  int _id;
+
+  /// A Future that will complete to the return value of [fn] once this task
+  /// finishes running.
+  Future get result => _resultCompleter.future;
+  final _resultCompleter = new Completer();
+
+  Task(fn(), this.queue, this.description) {
+    _id = this.queue.contents.length;
+    this.fn = () {
+      var future = new Future.immediate(null).then((_) => fn());
+      chainToCompleter(future, _resultCompleter);
+      return future;
+    };
+
+    // Make sure any error thrown by fn isn't top-leveled by virtue of being
+    // passed to the result future.
+    result.catchError((_) {});
+  }
+
+  String toString() => description == null ? "#$_id" : description;
+
+  /// Returns a detailed representation of [queue] with this task highlighted.
+  String generateTree() => queue.generateTree(this);
+}