feat(core): upgrade rxjs to 6.0.0-alpha.4 (#22573)

PR Close #22573

packages/router/src/recognize.ts

@@ -7,9 +7,7 @@
  */
 
 import {Type} from '@angular/core';
-import {Observable} from 'rxjs/Observable';
-import {Observer} from 'rxjs/Observer';
-import {of } from 'rxjs/observable/of';
+import {Observable, Observer, of } from 'rxjs';
 
 import {Data, ResolveData, Route, Routes} from './config';
 import {ActivatedRouteSnapshot, ParamsInheritanceStrategy, RouterStateSnapshot, inheritedParamsDataResolve} from './router_state';