Step one towards stable error messages with details:

pkg/polymer/test/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.
 
 library polymer.test.build.common;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
+import 'package:code_transformers/messages/build_logger.dart'
+  show LOG_EXTENSION;
 import 'package:polymer/src/build/common.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:unittest/unittest.dart';
 
 String idToString(AssetId id) => '${id.package}|${id.path}';
 AssetId idFromString(String s) {
   int index = s.indexOf('|');
   return new AssetId(s.substring(0, index), s.substring(index + 1));
 }
 
@@ skipping 221 matching lines @@
 
   'observe|lib/observe.dart':
       'library observe;\n'
       'export "src/metadata.dart";',
 
   'observe|lib/src/metadata.dart':
       'library observe.src.metadata;\n'
       'class ObservableProperty { const ObservableProperty(); }\n'
       'const observable = const ObservableProperty();\n',
 };