Emit a failing error code for failing validation.

test/lish/server_arg_overrides_publish_to_url_test.dart

Index: test/lish/server_arg_overrides_publish_to_url_test.dart
diff --git a/test/lish/server_arg_overrides_publish_to_url_test.dart b/test/lish/server_arg_overrides_publish_to_url_test.dart
index b5c9a14eb40b6b7918b02d0da978b2b5af2090b5..f165cb527bc5e4e616ec289ab13d6631454c105d 100644
--- a/test/lish/server_arg_overrides_publish_to_url_test.dart
+++ b/test/lish/server_arg_overrides_publish_to_url_test.dart
@@ -2,6 +2,7 @@
 // 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.
 
+import 'package:pub/src/exit_codes.dart' as exit_codes;
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
@@ -15,6 +16,7 @@ main() {
     d.dir(appPath, [d.pubspec(pkg)]).create();
 
     schedulePub(args: ["lish", "--dry-run", "--server", "http://arg.com"],
-        output: contains("http://arg.com"));
+        output: contains("http://arg.com"),
+        exitCode: exit_codes.DATA);
   });
 }