Properly handle barback logs after a transformer's been canceled.

pkg/barback/lib/src/transformer/base_transform.dart

Index: pkg/barback/lib/src/transformer/base_transform.dart
diff --git a/pkg/barback/lib/src/transformer/base_transform.dart b/pkg/barback/lib/src/transformer/base_transform.dart
index f76a6a481f9e3e436a23a6cdee2ff36af3b9a0ca..fe3ab3b6cdd3cf0708a66cc5cef89dbb9a49948b 100644
--- a/pkg/barback/lib/src/transformer/base_transform.dart
+++ b/pkg/barback/lib/src/transformer/base_transform.dart
@@ -49,7 +49,10 @@ abstract class BaseTransform {
       // If the log isn't already associated with an asset, use the primary.
       if (asset == null) asset = _node.info.primaryId;
       var entry = new LogEntry(_node.info, asset, level, message, span);
-      _onLogController.add(entry);
+
+      // The log controller can be closed while log entries are still coming in
+      // if the transformer is removed during [apply].
+      if (!_onLogController.isClosed) _onLogController.add(entry);
     });
   }