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

PR Close #22573

aio/content/examples/rx-library/src/operators.ts

@@ -1,12 +1,11 @@
 
-import { Observable } from 'rxjs/Observable';
-import 'rxjs/add/observable/of';
+import { Observable, of } from 'rxjs';
 
 // #docregion
 
 import { map } from 'rxjs/operators';
 
-const nums = Observable.of(1, 2, 3);
+const nums = of(1, 2, 3);
 
 const squareValues = map((val: number) => val * val);
 const squaredNums = squareValues(nums);