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>>[];
+  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;
+
+      _pending--;
+      _values.add(value);
+
+      if (_pending <= 0) {
+        completed = true;
+        _completer.complete(_values);
+      }
+    }).catchError((e) {
+      if (completed) return;
+
+      completed = true;
+      _completer.completeError(e.error, e.stackTrace);
+    }));
+
+    return task;
+  }
+
+  Future<List> get future => _completer.future;
+}
+