anchor.diff:    void addOutput(Asset output) {
       _call(_port, {
         'type': 'addOutput',
  -      'output': _serializeAsset(output)
  +      'output': serializeAsset(output)
       });
     }
   }
  
positive.code:   port.listen((wrappedMessage) {
      _respond(wrappedMessage, (message) {
        if (message['type'] == 'isPrimary') {
          return transformer.isPrimary(_deserializeAsset(message['asset']));
        } else {
          assert(message['type'] == 'apply');
          return transformer.apply(
  
negative.code:   assert(location['type'] == 'fixed');
    return new FixedLocation(location['offset'], location['sourceUrl'],
        location['line'], location['column']);
  }
  
  /// Converts [id] into a serializable map.
  