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

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

 // Copyright (c) 2014, 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.transformer.base_transform;
 
 import 'dart:async';
 
 import '../asset/asset_id.dart';
 import '../graph/transform_node.dart';
@@ skipping 31 matching lines @@
   TransformLogger get logger => _logger;
   TransformLogger _logger;
 
   BaseTransform(this._node) {
     _logger = new TransformLogger((asset, level, message, span) {
       if (level == LogLevel.ERROR) _loggedError = true;
 
       // 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);
     });
   }
 
   /// Consume a primary input so that it doesn't get processed by future
   /// phases or emitted once processing has finished.
   ///
   /// Normally each primary input will automatically be forwarded unless the
   /// transformer overwrites it by emitting an input with the same id. This
   /// allows the transformer to tell barback not to forward a primary input
   /// even if it's not overwritten.
@@ skipping 40 matching lines @@
   ///
   /// This will close any streams and release any resources that were allocated
   /// for the duration of the transformation. Unlike [done], this indicates that
   /// the transformation is no longer relevant; either it has returned, or
   /// something external has preemptively invalidated its results.
   void cancel() {
     done();
     transform._onLogController.close();
   }
 }