Improve barback/pub logging.

pkg/barback/lib/src/barback.dart

Index: pkg/barback/lib/src/barback.dart
diff --git a/pkg/barback/lib/src/barback.dart b/pkg/barback/lib/src/barback.dart
index 2393f1f004a27be18f096202a0aaa9bf239eb475..affc1b568cac077cb73d0b8ee6e22c85445d25f9 100644
--- a/pkg/barback/lib/src/barback.dart
+++ b/pkg/barback/lib/src/barback.dart
@@ -64,8 +64,8 @@ class Barback {
   /// 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].
   ///