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

PR Close #22573

aio/content/examples/rx-library/src/error-handling.ts

@@ -1,10 +1,9 @@
 
-import { Observable } from 'rxjs/Observable';
-import 'rxjs/add/observable/of';
+import { of } from 'rxjs';
 
 // #docregion
 
-import { ajax } from 'rxjs/observable/dom/ajax';
+import { ajax } from 'rxjs/ajax';
 import { map, catchError } from 'rxjs/operators';
 // Return "response" from the API. If an error happens,
 // return an empty array.
@@ -15,7 +14,7 @@ const apiData = ajax('/api/data').pipe(
     }
     return res.response;
   }),
-  catchError(err => Observable.of([]))
+  catchError(err => of([]))
 );
 
 apiData.subscribe({