fix barback and polymer build warnings

pkg/polymer/lib/src/build/common.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.
 
 /** Common methods used by transfomers. */
 library polymer.src.build.common;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ skipping 97 matching lines @@
       transform.getInput(id).then((_) => true).catchError((_) => false);
 }
 
 /** Create an [AssetId] for a [url] seen in the [source] asset. */
 // TODO(sigmund): delete once this is part of barback (dartbug.com/12610)
 AssetId resolve(AssetId source, String url, TransformLogger logger, Span span) {
   if (url == null || url == '') return null;
   var uri = Uri.parse(url);
   var urlBuilder = path.url;
   if (uri.host != '' || uri.scheme != '' || urlBuilder.isAbsolute(url)) {
-    logger.error('absolute paths not allowed: "$url"', span);
+    logger.error('absolute paths not allowed: "$url"', span: span);
     return null;
   }
 
   var package;
   var targetPath;
   var segments = urlBuilder.split(url);
   if (segments[0] == 'packages') {
     if (segments.length < 3) {
       logger.error("incomplete packages/ path. It should have at least 3 "
-          "segments packages/name/path-from-name's-lib-dir", span);
+          "segments packages/name/path-from-name's-lib-dir", span: span);
       return null;
     }
     package = segments[1];
     targetPath = urlBuilder.join('lib',
         urlBuilder.joinAll(segments.sublist(2)));
   } else if (segments[0] == 'assets') {
     if (segments.length < 3) {
       logger.error("incomplete assets/ path. It should have at least 3 "
-          "segments assets/name/path-from-name's-asset-dir", span);
+          "segments assets/name/path-from-name's-asset-dir", span: span);
     }
     package = segments[1];
     targetPath = urlBuilder.join('asset',
         urlBuilder.joinAll(segments.sublist(2)));
   } else {
     package = source.package;
     targetPath = urlBuilder.normalize(
         urlBuilder.join(urlBuilder.dirname(source.path), url));
   }
   return new AssetId(package, targetPath);
@@ skipping 30 matching lines @@
   return builder.relative(builder.join('/', id.path),
       from: builder.join('/', builder.dirname(sourceId.path)));
 }
 
 
 /** Convert system paths to asset paths (asset paths are posix style). */
 String _systemToAssetPath(String assetPath) {
   if (path.Style.platform != path.Style.windows) return assetPath;
   return path.posix.joinAll(path.split(assetPath));
 }