Use warning instead of error in a couple places. In polymer we are hitting

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: 'error: 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');
 
     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',
@@ skipping 18 matching lines @@
   final Function validation;
 
   Validator(this.validation);
 
   Future<bool> isPrimary(Asset input) => new Future.value(true);
 
   Future apply(Transform transform) {
     return new Future.value(validation(transform));
   }
 }