Consider errors logged by a transformer equivalent to thrown errors.

pkg/barback/test/utils.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.utils;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert' show Encoding;
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/cancelable_future.dart';
 import 'package:barback/src/utils.dart';
 import 'package:path/path.dart' as pathos;
 import 'package:scheduled_test/scheduled_test.dart';
 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';
 export 'transformer/create_asset.dart';
 export 'transformer/emit_nothing.dart';
 export 'transformer/lazy_bad.dart';
 export 'transformer/lazy_many_to_one.dart';
 export 'transformer/lazy_rewrite.dart';
 export 'transformer/many_to_one.dart';
 export 'transformer/mock.dart';
@@ skipping 60 matching lines @@
       (package, assets) => new AssetSet.from(assets));
 
   // Make sure that packages that have transformers but no assets are considered
   // by MockProvider to exist.
   for (var package in transformers.keys) {
     assetMap.putIfAbsent(package, () => new AssetSet());
   }
 
   _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;
 
   schedule(() => transformers.forEach(_barback.updateTransformers));
 
   // There should be one successful build after adding all the transformers but
   // before adding any sources.
   if (!transformers.isEmpty) buildShouldSucceed();
 }
 
@@ skipping 457 matching lines @@
 
   _MockAsset(this.id, this.contents);
 
   Future<String> readAsString({Encoding encoding}) =>
       new Future.value(contents);
 
   Stream<List<int>> read() => throw new UnimplementedError();
 
   String toString() => "MockAsset $id $contents";
 }