Support nested tasks in scheduled_test.

pkg/scheduled_test/lib/src/future_group.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 future_group;
+
+import 'dart:async';
+
+/// A completer that waits until all added [Future]s complete.
+// TODO(rnystrom): Copied from web_components. Remove from here when it gets
+// added to dart:core. (See #6626.)
+class FutureGroup<T> {
+  int _pending = 0;
+  Completer<List<T>> _completer = new Completer<List<T>>();
+  final List<Future<T>> futures = <Future<T>>[];

[Bob Nystrom, 2013/02/19 23:15:04]

Is there a reason to store these?

[nweiz, 2013/02/20 00:23:12]

I don't want to change this code here without changing it everywhere else it's
used. These are good comments, but they really belong on the code review that
comes out of issue 6626.

+  bool completed = false;
+
+  final List<T> _values = <T>[];
+
+  /// Wait for [task] to complete.
+  Future<T> add(Future<T> task) {
+    if (completed) {
+      throw new StateError("The FutureGroup has already completed.");
+    }
+
+    _pending++;
+    futures.add(task.then((value) {
+      if (completed) return;

[Bob Nystrom, 2013/02/19 23:15:04]

It seems weird to swallow this. Doesn't this indicate something fishy happened?

[nweiz, 2013/02/20 00:23:12]

In this code, the futures passed to FutureGroup are guaranteed never to pass
errors through. For other code, see comment above.

+
+      _pending--;
+      _values.add(value);
+
+      if (_pending <= 0) {
+        completed = true;
+        _completer.complete(_values);
+      }
+    }).catchError((e) {
+      if (completed) return;

[Bob Nystrom, 2013/02/19 23:15:04]

Ditto here.

+
+      completed = true;

[Bob Nystrom, 2013/02/19 23:15:04]

It seems like every place we use a completer we end up needing this flag. File a
bug to ask them to have the new lib_v2 completer expose this? It does store it
internally.

[nweiz, 2013/02/20 00:23:12]

Filed as issue 8638.

+      _completer.completeError(e.error, e.stackTrace);
+    }));
+
+    return task;
+  }
+
+  Future<List> get future => _completer.future;
+}
+