anchor.diff:  import "dart:core";
   import "dart:uri";
   import "dart:html";
  -import 'package:web_ui/watcher.dart' as watchers;
   import 'package:web_ui/observe.dart';
   import 'package:delegate/delegate.dart';
   
  
positive.code:       this.activeWatchDisposer();
      }
  
      this.activeWatchDisposer = this.watch(match[1], (e) {
        this.history.replaceState(null, null,
          this.router.routePath(this.activeRoute, e.newValue)
        );
      });
  
negative.code:   var activeWatchDisposer;
  
    PageNavigator(
      this.observe,
      this.history,
      this.router,
      this.views,
  