Fix type errors detected with --error-on-bad-type.

pkg/barback/lib/src/transform_node.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.transform_node;
 
 import 'dart:async';
+import 'package:source_maps/span.dart' show Span;
 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'asset_set.dart';
 import 'barback_logger.dart';
 import 'errors.dart';
 import 'phase.dart';
 import 'transform.dart';
 import 'transformer.dart';
@@ skipping 193 matching lines @@
   }
 
   void _log(AssetId asset, LogLevel level, String message, Span span) {
     // If the log isn't already associated with an asset, use the primary.
     if (asset == null) asset = primary.id;
     var info = new TransformInfo(transformer, primary.id);
     var entry = new LogEntry(info, asset, level, message, span);
     _onLogController.add(entry);
   }
 }