Step one towards stable error messages with details:

pkg/code_transformers/test/assets_test.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.
 
 library code_transformers.test.assets_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:code_transformers/assets.dart';
@@ skipping 48 matching lines @@
         result: new AssetId('foo', 'lib/foo.dart'));
 
     testAssetUri('resolves relative packages paths',
         source: new AssetId('a', 'web/main.dart'),
         uri: 'packages/foo/foo.dart',
         result: new AssetId('foo', 'lib/foo.dart'));
 
     testAssetUri('does not allow packages from non-dart lib files',
         source: new AssetId('a', 'lib/index.html'),
         uri: 'packages/foo/bar',
-        message: 'warning: Invalid url to reach to another package: '
+        message: 'warning: Invalid URL to reach to another package: '
             'packages/foo/bar. Path reaching to other packages must first '
-            'reach up all the way to the packages folder. For example, try '
-            'changing the url above to: ../../packages/foo/bar');
+            'reach up all the way to the packages directory. For example, try '
+            'changing the URL to: ../../packages/foo/bar');
 
     testAssetUri('allows relative packages from non-dart lib files',
         source: new AssetId('a', 'lib/index.html'),
         uri: '../../packages/foo/bar',
         result: new AssetId('foo', 'lib/bar'));
 
     testAssetUri('does not allow package: imports from non-dart files',
         source: new AssetId('a', 'lib/index.html'),
         uri: 'package:foo/bar.dart',
         message: 'warning: absolute paths not allowed: "package:foo/bar.dart"');
@@ skipping 14 matching lines @@
 
 class Validator extends Transformer {
   final Function validation;
 
   Validator(this.validation);
 
   Future apply(Transform transform) {
     return new Future.value(validation(transform));
   }
 }