Fixes code_transformers after the latest analyzer changes

pkg/code_transformers/lib/src/test_harness.dart

 // Copyright (c) 2014, 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.
 
 /// Utilities for creating unit tests of Barback transformers.
 library code_transformers.src.test_harness;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ skipping 99 matching lines @@
     return barback.results.first.then((_) {
       if (files == null) return null;
       var futures = [];
       files.forEach((k, v) {
         futures.add(check(k, v));
       });
       return Future.wait(futures);
     }).then((_) {
       // We only check messages when an expectation is provided.
       if (messages == null) return;
-      expect(messages.length, messagesSeen,
+      expect(messagesSeen, messages.length,
           reason: 'less messages than expected');
     });
   }
 }