Improve barback/pub logging.

pkg/barback/lib/src/barback.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.barback;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 46 matching lines @@
 
   /// A stream that emits any errors from the graph or the transformers.
   ///
   /// This emits errors as they're detected. If an error occurs in one part of
   /// the graph, unrelated parts will continue building.
   ///
   /// This will not emit programming errors from barback itself. Those will be
   /// emitted through the [results] stream's error channel.
   Stream get errors => _graph.errors;
 
-  Barback(PackageProvider provider)
-      : _graph = new PackageGraph(provider);
+  Barback(PackageProvider provider, {BarbackLogger logger})

[nweiz, 2013/10/16 19:41:27]

I'm not a big fan of using injection here. What do you think of providing a log
stream (maybe replacing the [errors] stream), and having it log to stdout if
there are no listeners?

[Bob Nystrom, 2013/10/28 23:45:56]

I thought about that (and it is using streams internally), but I'm not a fan of
there being different behavior based on whether or not a stream has listeners.
I'm up for discussing this though, I could be convinced.

[nweiz, 2013/10/29 00:40:13]

There's some precedent for this in that buffered streams will only top-level
exceptions if they have a listener. I don't think triggering based on a listener
is something we want to do all the time, but I think this is a case where it
makes sense, especially since the behavioral change when a listener added isn't
something that affects the actual code -- just what the end user sees.

[Bob Nystrom, 2013/10/29 18:29:45]

Done.

+      : _graph = new PackageGraph(provider, logger: logger);
 
   /// Gets the asset identified by [id].
   ///
   /// If [id] is for a generated or transformed asset, this will wait until
   /// it has been created and return it. If the asset cannot be found, throws
   /// [AssetNotFoundException].
   Future<Asset> getAssetById(AssetId id) {
     return _graph.getAssetNode(id).then((node) {
       if (node == null) throw new AssetNotFoundException(id);
       return node.asset;
@@ skipping 23 matching lines @@
   ///
   /// To the extent that [transformers] is similar to the previous transformer
   /// phases for [package], the existing asset graph will be preserved.
   ///
   /// Elements of the inner iterable of [transformers] must be either
   /// [Transformer]s or [TransformerGroup]s.
   void updateTransformers(String package,
           Iterable<Iterable> transformers) =>
       _graph.updateTransformers(package, transformers);
 }