Step one towards stable error messages with details:

pkg/polymer/lib/src/build/build_filter.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.
 
 /// Final phase of the polymer transformation: removes any files that are not
 /// needed for deployment.
 library polymer.src.build.build_filter;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
+import 'package:code_transformers/messages/build_logger.dart';
 import 'common.dart';
 
 /// Removes any files not needed for deployment, such as internal build
 /// artifacts and non-entry HTML files.
 class BuildFilter extends Transformer with PolymerTransformer {
   final TransformOptions options;
   BuildFilter(this.options);
 
   isPrimary(AssetId id) {
     // nothing is filtered in debug mode
     return options.releaseMode &&
       // TODO(sigmund): remove this exclusion once we have dev_transformers
       // (dartbug.com/14187)
       !id.path.startsWith('lib/') &&
       // may filter non-entry HTML files and internal artifacts
       (id.extension == '.html' || id.extension == _DATA_EXTENSION) &&
       // keep any entry points
       !options.isHtmlEntryPoint(id);
   }
 
   apply(Transform transform) {
     transform.consumePrimary();
     if (transform.primaryInput.id.extension == _DATA_EXTENSION) {
       return null;
     }
-    return readPrimaryAsHtml(transform).then((document) {
+    var logger = new BuildLogger(transform,
+        convertErrorsToWarnings: !options.releaseMode);
+    return readPrimaryAsHtml(transform, logger).then((document) {
       // Keep .html files that don't use polymer, since the app developer might
       // have non-polymer entrypoints.
       if (document.querySelectorAll('polymer-element').isEmpty) {
         transform.addOutput(transform.primaryInput);
       }
     });
   }
 }
 
 const String _DATA_EXTENSION = '._data';