anchor.diff:      }
       var block = new _SyncBlock._(name, _getTraceClock());
       if (arguments is Map) {
  -      block.arguments.addAll(arguments);
  +      block._appendArguments(arguments);
       }
       _stack.add(block);
     }
  
positive.code:     }
      var block = new _AsyncBlock._(name, _taskId);
      if (arguments is Map) {
        block.arguments.addAll(arguments);
      }
      _stack.add(block);
      block._start();
  
negative.code:                      category,
                       name,
                       _argumentsAsJson(null));
    }
  
    void _appendArguments(Map arguments) {
      if (_arguments == null) {
        _arguments = {};
      }
      _arguments.addAll(arguments);
    }
  }
  
  