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

PR Close #22573

aio/content/examples/styleguide/src/07-03/app/heroes/shared/hero.service.ts

@@ -1,8 +1,7 @@
 // #docregion
 import { Injectable } from '@angular/core';
 
-import { Observable } from 'rxjs/Observable';
-import 'rxjs/add/observable/of';
+import { Observable, of } from 'rxjs';
 
 import { Hero } from './hero.model';
 
@@ -10,6 +9,6 @@ import { Hero } from './hero.model';
 export class HeroService {
   getHeroes() {
     let heroes: Hero[] = [];
-    return Observable.of(heroes);
+    return of(heroes);
   }
 }