Consider errors logged by a transformer equivalent to thrown errors.

pkg/barback/test/package_graph/errors_test.dart

Index: pkg/barback/test/package_graph/errors_test.dart
diff --git a/pkg/barback/test/package_graph/errors_test.dart b/pkg/barback/test/package_graph/errors_test.dart
index d1e8ea3726f579dfff1e24bfce519b9650eed7e7..8cc134be2111c1fdef12f8777a57ed722d973a53 100644
--- a/pkg/barback/test/package_graph/errors_test.dart
+++ b/pkg/barback/test/package_graph/errors_test.dart
@@ -152,4 +152,18 @@ main() {
     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"))
+    ]);
+  });
 }