Pass logs through transformer groups.

pkg/barback/lib/src/phase.dart

Index: pkg/barback/lib/src/phase.dart
diff --git a/pkg/barback/lib/src/phase.dart b/pkg/barback/lib/src/phase.dart
index d7123542df97294ecf43fdda65b2557560dc7bb3..2136a1281e94d65e04d1914441a4b94ec7faeb90 100644
--- a/pkg/barback/lib/src/phase.dart
+++ b/pkg/barback/lib/src/phase.dart
@@ -116,8 +116,10 @@ class Phase {
     _onDirtyPool.add(_onDirtyController.stream);
 
     for (var group in transformers.where((op) => op is TransformerGroup)) {
-      _groups[group] = new GroupRunner(cascade, group);
-      _onDirtyPool.add(_groups[group].onDirty);
+      var runner = new GroupRunner(cascade, group);
+      _groups[group] = runner;
+      _onDirtyPool.add(runner.onDirty);
+      _onLogPool.add(runner.onLog);
     }
   }
 
@@ -206,6 +208,7 @@ class Phase {
       var runner = new GroupRunner(cascade, added);
       _groups[added] = runner;
       _onDirtyPool.add(runner.onDirty);
+      _onLogPool.add(runner.onLog);
       for (var input in _inputs.values) {
         runner.addInput(input.input);
       }