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