Ensure that errors have stack traces attached.

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
@@ skipping 16 matching lines @@
     futures.add(task.then((value) {
       if (completed) return;
 
       _pending--;
       _values.add(value);
 
       if (_pending <= 0) {
         completed = true;
         _completer.complete(_values);
       }
-    }).catchError((error) {
+    }).catchError((error, stackTrace) {
       if (completed) return;
 
       completed = true;
-      _completer.completeError(error);
+      _completer.completeError(error, stackTrace);
     }));
 
     return task;
   }
 
   Future<List> get future => _completer.future;
 }