Consider errors logged by a transformer equivalent to thrown errors.

pkg/barback/test/package_graph/errors_test.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 barback.test.package_graph.source_test;
 
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
@@ skipping 134 matching lines @@
     ]);
   });
 
   test("an asset isn't passed through a transformer with an error", () {
     initGraph(["app|foo.txt",], {"app": [[new BadTransformer([])]]});
 
     updateSources(["app|foo.txt"]);
     expectNoAsset("app|foo.txt");
     buildShouldFail([isTransformerException(equals(BadTransformer.ERROR))]);
   });
+
+  test("a transformer that logs errors shouldn't produce output", () {
+    initGraph(["app|foo.txt"], {"app": [
+      [new BadLogTransformer(["app|out.txt"])]
+    ]});
+
+    updateSources(["app|foo.txt"]);
+    expectNoAsset("app|foo.txt");
+    expectNoAsset("app|out.txt");
+    buildShouldFail([
+      isTransformerException(equals("first error")),
+      isTransformerException(equals("second error"))
+    ]);
+  });
 }