Ensure that errors have stack traces attached.

pkg/scheduled_test/lib/src/scheduled_server/safe_http_server.dart

Index: pkg/scheduled_test/lib/src/scheduled_server/safe_http_server.dart
diff --git a/pkg/scheduled_test/lib/src/scheduled_server/safe_http_server.dart b/pkg/scheduled_test/lib/src/scheduled_server/safe_http_server.dart
index a916c1d84daf7dffd402525617f06f48d6c73885..9e1eeb29efcd755aba0744072198c15eb4deb914 100644
--- a/pkg/scheduled_test/lib/src/scheduled_server/safe_http_server.dart
+++ b/pkg/scheduled_test/lib/src/scheduled_server/safe_http_server.dart
@@ -143,5 +143,6 @@ class _HttpResponseWrapper implements HttpResponse {
     _inner.writeAll(objects, separator);
   void writeCharCode(int charCode) => _inner.writeCharCode(charCode);
   void writeln([Object obj = ""]) => _inner.writeln(obj);
-  void addError(error) => _inner.addError(error);
+  void addError(error, [StackTrace stackTrace]) =>
+      _inner.addError(error, stackTrace);
 }