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

PR Close #22573

aio/content/examples/hierarchical-dependency-injection/src/app/heroes-list.component.ts

@@ -1,6 +1,6 @@
 // #docregion
 import { Component }  from '@angular/core';
-import { Observable } from 'rxjs/Observable';
+import { Observable } from 'rxjs';
 
 import { Hero, HeroTaxReturn } from './hero';
 import { HeroesService }       from './heroes.service';

aio/content/examples/hierarchical-dependency-injection/src/app/heroes.service.ts

@@ -1,7 +1,6 @@
 import { Injectable } from '@angular/core';
 
-import { Observable } from 'rxjs/Observable';
-import { Observer } from 'rxjs/Observer';
+import { Observable, Observer } from 'rxjs';
 
 import { Hero, HeroTaxReturn } from './hero';
 

aio/content/examples/hierarchical-dependency-injection/src/app/villains-list.component.ts

@@ -1,6 +1,6 @@
 // #docregion
 import { Component }  from '@angular/core';
-import { Observable } from 'rxjs/Observable';
+import { Observable } from 'rxjs';
 
 import { Villain, VillainsService } from './villains.service';
 

aio/content/examples/hierarchical-dependency-injection/src/app/villains.service.ts

@@ -1,6 +1,6 @@
 import { Injectable } from '@angular/core';
 
-import { of }         from 'rxjs/observable/of';
+import { of }         from 'rxjs';
 
 export interface Villain { id: number; name: string; }