fix barback and polymer build warnings

pkg/scheduled_test/test/scheduled_process_test.dart

 // Copyright (c) 2013, 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.
 
 library scheduled_process_test;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 366 matching lines @@
 
           void main() {
             $script
           }
           ''').then((file) => file.path);
     });
   }, 'write script file');
 
   currentSchedule.onComplete.schedule(() {
     return tempDir.catchError((_) => null).then((dir) {
-      if (dir == null) return;
+      if (dir == null) return null;
       return new Directory(dir).delete(recursive: true);
     });
   }, 'clean up temp dir');
 
   return new ScheduledProcess.start(Platform.executable,
       ['--checked', dartPath]);
 }