fix barback and polymer build warnings

pkg/polymer/lib/src/build/script_compactor.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.
 
 /** Transfomer that combines multiple dart script tags into a single one. */
 library polymer.src.build.script_compactor;
 
 import 'dart:async';
 import 'dart:convert';
 
@@ skipping 56 matching lines @@
             tag.remove();
             continue;
           }
           mainLibraryId = resolve(id, src, logger, tag.sourceSpan);
           mainScriptTag = tag;
         }
 
         if (mainScriptTag == null) {
           // We didn't find any main library, nothing to do.
           transform.addOutput(transform.primaryInput);
-          return;
+          return null;
         }
 
         var bootstrapId = id.addExtension('_bootstrap.dart');
-        mainScriptTag.attributes['src'] = 
+        mainScriptTag.attributes['src'] =
             path.url.basename(bootstrapId.path);
 
         // TODO(sigmund): stop renaming the file (see dartbug.com/14554)
         var modifiedMainId = mainLibraryId.addExtension('_modified.dart');
 
         libraries.add(modifiedMainId);
         var urls = libraries.map((id) => assetUrlFor(id, bootstrapId, logger))
             .where((url) => url != null).toList();
         var buffer = new StringBuffer()..writeln(MAIN_HEADER);
         int i = 0;
@@ skipping 20 matching lines @@
       });
     });
   }
 }
 
 const MAIN_HEADER = """
 library app_bootstrap;
 
 import 'package:polymer/polymer.dart';
 """;