Miscellaneous small improvements for scheduled_test.

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 7f1856b66c1987bfbe1494bb8cf10df8fe61e328..a1f8a969b0faa0946325acb5131c4a7b7f8dff46 100644
--- a/pkg/scheduled_test/lib/src/schedule_error.dart
+++ b/pkg/scheduled_test/lib/src/schedule_error.dart
@@ -48,6 +48,14 @@ class ScheduleError extends AsyncError {
       error = error.error;
     }
 
+    if (stackTrace == null) {
+      try {
+        throw '';
+      } catch (_, thrownStackTrace) {
+        stackTrace = thrownStackTrace;
+      }
+    }
+
     return new ScheduleError(schedule, error, stackTrace, cause);
   }