report error message when import urls are broken.

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 60 matching lines @@
 
   Future<Document> readAsHtml(AssetId id, Transform transform) {
     var primaryId = transform.primaryInput.id;
     var url = (id.package == primaryId.package) ? id.path
         : assetUrlFor(id, primaryId, transform.logger, allowAssetUrl: true);
     return transform.readInputAsString(id).then((content) {
       return _parseHtml(content, url, transform.logger,
         checkDocType: options.isHtmlEntryPoint(id));
     });
   }
+
+  Future<bool> assetExists(AssetId id, Transform transform) =>
+      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);
@@ skipping 59 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));
 }