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

pkg/barback/lib/src/transform.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;
 
 import 'dart:async';
 import 'dart:convert';
+import 'package:source_maps/span.dart' show Span;

[nweiz, 2013/10/22 23:18:23]

Style nit: add a newline before this. Also, we tend not to use "show" unless
it's specifically necessary in a file. Same goes fro transform_node.dart.

[Siggi Cherem (dart-lang), 2013/10/22 23:41:18]

Done.

 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'asset_set.dart';
+import 'barback_logger.dart';
 import 'errors.dart';
 import 'transform_logger.dart';
 import 'transform_node.dart';
 import 'utils.dart';
 
 typedef void LogFunction(AssetId asset, LogLevel level, String message,
                          Span span);
 
 /// Creates a [Transform] by forwarding to the private constructor.
 ///
@@ skipping 73 matching lines @@
 
   /// Stores [output] as the output created by this transformation.
   ///
   /// A transformation can output as many assets as it wants.
   void addOutput(Asset output) {
     // TODO(rnystrom): This should immediately throw if an output with that ID
     // has already been created by this transformer.
     _outputs.add(output);
   }
 }