Step one towards stable error messages with details:

pkg/code_transformers/lib/assets.dart

 // Copyright (c) 2014, 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 for dealing with asset IDs.
 library code_transformers.assets;
 
 import 'dart:math' show min, max;
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
 import 'package:source_span/source_span.dart';
 
+import 'messages/build_logger.dart';
+import 'src/messages.dart';
+
 /// Create an [AssetId] for a [url] seen in the [source] asset.
 ///
 /// By default this is used to resolve relative urls that occur in HTML assets,
 /// including cross-package urls of the form "packages/foo/bar.html". Dart
 /// "package:" urls are not resolved unless [source] is Dart file (has a .dart
 /// extension).
 ///
 /// This function returns null if [url] can't be resolved. We log a warning
 /// message in [logger] if we know the reason why resolution failed. This
 /// happens, for example, when using incorrect paths to reach into another
 /// package, or when [errorsOnAbsolute] is true and the url seems to be
 /// absolute.
 // TODO(sigmund): delete once this is part of barback (dartbug.com/12610)
 AssetId uriToAssetId(AssetId source, String url, TransformLogger logger,
     SourceSpan span, {bool errorOnAbsolute: true}) {
   if (url == null || url == '') return null;
   var uri = Uri.parse(url);
   var urlBuilder = path.url;
   if (uri.host != '' || uri.scheme != '' || urlBuilder.isAbsolute(url)) {
     if (source.extension == '.dart' && uri.scheme == 'package') {
       var index = uri.path.indexOf('/');
       if (index != -1) {
         return new AssetId(uri.path.substring(0, index),
             'lib${uri.path.substring(index)}');
       }
     }
 
     if (errorOnAbsolute) {
-      logger.warning('absolute paths not allowed: "$url"', span: span);
+      var msg = NO_ABSOLUTE_PATHS.create({'url': url});
+      logger.warning(logger is BuildLogger ? msg : msg.snippet, span: span);
     }
     return null;
   }
 
   var targetPath = urlBuilder.normalize(
       urlBuilder.join(urlBuilder.dirname(source.path), url));
   var segments = urlBuilder.split(targetPath);
   var sourceSegments = urlBuilder.split(source.path);
   assert (sourceSegments.length > 0);
   var topFolder = sourceSegments[0];
@@ skipping 21 matching lines @@
       // folder lives (otherwise the app would not work in Dartium). Since
       // [targetPath] has been normalized, "packages" or "assets" should be at
       // index 1.
       return _extractOtherPackageId(1, segments, logger, span);
     } else {
       var prefix = segments[index];
       var fixedSegments = [];
       fixedSegments.addAll(sourceSegments.map((_) => '..'));
       fixedSegments.addAll(segments.sublist(index));
       var fixedUrl = urlBuilder.joinAll(fixedSegments);
-      logger.warning('Invalid url to reach to another package: $url. Path '
-          'reaching to other packages must first reach up all the '
-          'way to the $prefix folder. For example, try changing the url above '
-          'to: $fixedUrl', span: span);
+      var msg = INVALID_URL_TO_OTHER_PACKAGE.create(
+          {'url': url, 'prefix': prefix, 'fixedUrl': fixedUrl});
+      logger.warning(logger is BuildLogger ? msg : msg.snippet, span: span);
       return null;
     }
   }
 
   // Otherwise, resolve as a path in the same package.
   return new AssetId(source.package, targetPath);
 }
 
 AssetId _extractOtherPackageId(int index, List segments,
     TransformLogger logger, SourceSpan span) {
   if (index >= segments.length) return null;
   var prefix = segments[index];
   if (prefix != 'packages' && prefix != 'assets') return null;
   var folder = prefix == 'packages' ? 'lib' : 'asset';
   if (segments.length < index + 3) {
-    logger.warning("incomplete $prefix/ path. It should have at least 3 "
-        "segments $prefix/name/path-from-name's-$folder-dir", span: span);
+    var msg = INVALID_PREFIX_PATH.create({'prefix': prefix, 'folder': folder});
+    logger.warning(logger is BuildLogger ? msg : msg.snippet, span: span);
     return null;
   }
   return new AssetId(segments[index + 1],
       path.url.join(folder, path.url.joinAll(segments.sublist(index + 2))));
 }