anchor.diff:   * Registeres a [view] for a particular [Route] identified by [route_name] in [Router].
    * It is not allowed to override already registered view.
    */
  -  void registerView(String routeName, View view){
  -    if(_views.containsKey(routeName)){
  +  void registerView(String routeName, View view) {
  +    if(_views.containsKey(routeName)) {
         throw new ArgumentError("Route name '$routeName' already in use in PageNavigator.");
       }
       _views[routeName] = view;
  
positive.code:     // Decode [url] parameters and fill them into [Map].
      Map result = new Map();
      for (var i = 0; i < _variables.length; i++) {
        result[_variables[i]] = Uri.decodeComponent(match.group(i+1));
      }
  
      return result;
  
negative.code:  *
   * Use flag [pushState] to push the new url to browser history.
   */
    void navigate(String routeName, Map parameters, {bool pushState: false}) {
      //== prepare variables
      var path = _router.routePath(routeName, parameters);
  
      if(!_views.containsKey(routeName)) {
        throw new ArgumentError("View not found for '$routeName'");
      }
  
  