Fix the initialization of PendingCallbacks in scheduled_test.

pkg/scheduled_test/lib/src/schedule.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 schedule;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 485 matching lines @@
     _totalCallbacks++;
     var trace = new Trace.current();
     var pendingCallback = new PendingCallback._(() {
       var fullDescription = description;
       if (fullDescription == null) {
         fullDescription = "Out-of-band operation #${_totalCallbacks}";
       }
 
       var stackString = prefixLines(terseTraceString(trace));
       fullDescription += "\n\nStack trace:\n$stackString";
+      return fullDescription;
     });
     _pendingCallbacks.add(pendingCallback);
 
     return (arg) {
       try {
         return fn(arg);
       } catch (e, stackTrace) {
         var error = new ScheduleError.from(
             _schedule, e, stackTrace: stackTrace);
         if (_timedOut()) {
@@ skipping 90 matching lines @@
   /// The string description of the callback.
   String get description {
     if (_description == null) _description = _thunk();
     return _description;
   }
 
   String toString() => description;
 
   PendingCallback._(this._thunk);
 }