anchor.diff:    Endpoint _pbEndpoint;
   
     Node(this.host, {this.httpPort: 8098, this.pbPort: 8087}) {
  -    _httpEndpoint = new Endpoint(host, httpPort);
  -    _pbEndpoint = new Endpoint(host, pbPort);
  +    assert(httpPort != null || pbPort != null);
  +    _httpEndpoint = httpPort == null ? null : new Endpoint(host, httpPort);
  +    _pbEndpoint = pbPort == null ? null : new Endpoint(host, pbPort);
     }
   
     bool operator ==(Node other) =>
  
positive.code:     if (_nodes.contains(node)) {
        return;
      }
      _httpPool.join(node._httpEndpoint);
      _pbPool.join(node._pbEndpoint);
      _nodes.add(node);
    }
  
  
negative.code:     if (!_nodes.contains(node)) {
        return;
      }
      if (node._httpEndpoint != null) {
        _httpPool.leave(node._httpEndpoint);
      }
      if (node._pbEndpoint != null) {
        _pbPool.leave(node._pbEndpoint);
      }
      _nodes.remove(node);
    }
  
  