Always capture stack traces when running pub tests.

pkg/scheduled_test/lib/src/schedule_error.dart

Index: pkg/scheduled_test/lib/src/schedule_error.dart
diff --git a/pkg/scheduled_test/lib/src/schedule_error.dart b/pkg/scheduled_test/lib/src/schedule_error.dart
index 09bb35095daebf429c384a618e37ca376731395b..8dc26650c764577e4c63da60bb8bfb4a7c49fa64 100644
--- a/pkg/scheduled_test/lib/src/schedule_error.dart
+++ b/pkg/scheduled_test/lib/src/schedule_error.dart
@@ -33,7 +33,7 @@ class ScheduleError {
 
   /// The descriptions of out-of-band callbacks that were pending when this
   /// error occurred.
-  final Iterable<String> pendingCallbacks;
+  final Iterable<PendingCallback> pendingCallbacks;
 
   /// The state of the schedule at the time the error was detected.
   final ScheduleState _stateWhenDetected;
@@ -69,7 +69,7 @@ class ScheduleError {
         schedule = schedule,
         task = schedule.currentTask,
         queue = schedule.currentQueue,
-        pendingCallbacks = schedule.currentQueue == null ? <String>[]
+        pendingCallbacks = schedule.currentQueue == null ? <PendingCallback>[]
             : schedule.currentQueue.pendingCallbacks.toList(),
         _stateWhenDetected = schedule.state;
 
@@ -112,7 +112,7 @@ class ScheduleError {
       result.write("\n\n");
       result.writeln("Pending out-of-band callbacks:");
       for (var callback in pendingCallbacks) {
-        result.writeln(prefixLines(callback, firstPrefix: "* "));
+        result.writeln(prefixLines(callback.toString(), firstPrefix: "* "));
       }
     }