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

PR Close #22573

packages/common/http/src/xhr.ts

@@ -7,8 +7,7 @@
  */
 
 import {Injectable} from '@angular/core';
-import {Observable} from 'rxjs/Observable';
-import {Observer} from 'rxjs/Observer';
+import {Observable, Observer} from 'rxjs';
 
 import {HttpBackend} from './backend';
 import {HttpHeaders} from './headers';