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

PR Close #22573

aio/content/examples/component-interaction/src/app/astronaut.component.ts

@@ -2,7 +2,7 @@
 import { Component, Input, OnDestroy } from '@angular/core';
 
 import { MissionService } from './mission.service';
-import { Subscription }   from 'rxjs/Subscription';
+import { Subscription }   from 'rxjs';
 
 @Component({
   selector: 'app-astronaut',

aio/content/examples/component-interaction/src/app/mission.service.ts

@@ -1,6 +1,6 @@
 // #docregion
 import { Injectable } from '@angular/core';
-import { Subject }    from 'rxjs/Subject';
+import { Subject }    from 'rxjs';
 
 @Injectable()
 export class MissionService {