Consider errors logged by a transformer equivalent to thrown errors.

pkg/barback/test/utils.dart

Index: pkg/barback/test/utils.dart
diff --git a/pkg/barback/test/utils.dart b/pkg/barback/test/utils.dart
index 9597a21aca56c34bb4f7a30d2b974095196eaa1a..465d7e9131b65ca27c7f8d98a01792898a595e69 100644
--- a/pkg/barback/test/utils.dart
+++ b/pkg/barback/test/utils.dart
@@ -17,6 +17,7 @@ import 'package:stack_trace/stack_trace.dart';
 import 'package:unittest/compact_vm_config.dart';
 
 export 'transformer/bad.dart';
+export 'transformer/bad_log.dart';
 export 'transformer/check_content.dart';
 export 'transformer/check_content_and_rename.dart';
 export 'transformer/conditionally_consume_primary.dart';
@@ -97,6 +98,8 @@ void initGraph([assets,
 
   _provider = new MockProvider(assetMap);
   _barback = new Barback(_provider);
+  // Add a dummy listener to the log so it doesn't print to stdout.
+  _barback.log.listen((_) {});
   _nextBuildResult = 0;
   _nextLog = 0;