Miscellaneous small improvements for scheduled_test.

pkg/scheduled_test/lib/src/schedule_error.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_error;
 
 import 'dart:async';
 
 import 'schedule.dart';
 import 'task.dart';
@@ skipping 30 matching lines @@
     if (error is ScheduleError) return error;
 
     if (error is AsyncError) {
       // Overwrite the explicit stack trace, because it probably came from a
       // rethrow in the first place.
       stackTrace = error.stackTrace;
       if (cause == null) cause = error.cause;
       error = error.error;
     }
 
+    if (stackTrace == null) {
+      try {
+        throw '';
+      } catch (_, thrownStackTrace) {
+        stackTrace = thrownStackTrace;
+      }
+    }
+
     return new ScheduleError(schedule, error, stackTrace, cause);
   }
 
   ScheduleError(Schedule schedule, error, stackTrace, AsyncError cause)
       : super.withCause(error, stackTrace, cause),
         schedule = schedule,
         task = schedule.currentTask,
         queue = schedule.currentQueue,
         pendingCallbacks = schedule.currentQueue == null ? <String>[]
             : schedule.currentQueue.pendingCallbacks.toList(),
@@ skipping 37 matching lines @@
       result.write("\n\n");
       result.writeln("Pending out-of-band callbacks:");
       for (var callback in pendingCallbacks) {
         result.writeln("* $callback");
       }
     }
 
     return result.toString().trim();
   }
 }