anchor.diff:    final int _taskId;
     /// An (optional) set of arguments which will be serialized to JSON and
     /// associated with this block.
  -  final Map arguments = {};
  +  Map _arguments;
   
     _AsyncBlock._(this.name, this._taskId);
   
  
positive.code:                      'b',
                       category,
                       name,
                       _argumentsAsJson(arguments));
    }
  
    // Emit the finish event.
  
negative.code:     }
      var block = new _AsyncBlock._(name, _taskId);
      if (arguments is Map) {
        block._appendArguments(arguments);
      }
      _stack.add(block);
      block._start();
  