Consider errors logged by a transformer equivalent to thrown errors.

pkg/barback/test/transformer/bad_log.dart

Index: pkg/barback/test/transformer/bad_log.dart
diff --git a/pkg/barback/test/transformer/bad.dart b/pkg/barback/test/transformer/bad_log.dart
similarity index 60%
copy from pkg/barback/test/transformer/bad.dart
copy to pkg/barback/test/transformer/bad_log.dart
index 49046ca5b45aed5e05dda27f5de0121ecd6aac9a..9e49f8b1e8e6513c14ca399465061ca14c9a9232 100644
--- a/pkg/barback/test/transformer/bad.dart
+++ b/pkg/barback/test/transformer/bad_log.dart
@@ -1,8 +1,8 @@
-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// 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 barback.test.transformer.bad;
+library barback.test.transformer.bad_log;
 
 import 'dart:async';
 
@@ -11,29 +11,25 @@ import 'package:barback/src/utils.dart';
 
 import 'mock.dart';
 
-/// A transformer that throws an exception when run, after generating the
-/// given outputs.
-class BadTransformer extends MockTransformer {
-  /// The error it throws.
-  static const ERROR = "I am a bad transformer!";
-
+/// A transformer that logs an error when run, Before generating the given
+/// outputs.
+class BadLogTransformer extends MockTransformer {
   /// The list of asset names that it should output.
   final List<String> outputs;
 
-  BadTransformer(this.outputs);
+  BadLogTransformer(this.outputs);
 
   Future<bool> doIsPrimary(Asset asset) => new Future.value(true);
 
   Future doApply(Transform transform) {
     return newFuture(() {
-      // Create the outputs first.
+      transform.logger.error("first error");
+      transform.logger.error("second error");
+
       for (var output in outputs) {
         var id = new AssetId.parse(output);
         transform.addOutput(new Asset.fromString(id, output));
       }
-
-      // Then fail.
-      throw ERROR;
     });
   }
 }