refactor(): use const and let instead of var

modules/@angular/examples/facade/ts/async/observable.ts

@@ -10,8 +10,8 @@
 import {Observable} from 'rxjs/Observable';
 import {Subscriber} from 'rxjs/Subscriber';
 
-var obs = new Observable<number>((obs: Subscriber<number>) => {
-  var i = 0;
+const obs = new Observable<number>((obs: Subscriber<number>) => {
+  let i = 0;
   setInterval(() => { obs.next(++i); }, 1000);
 });
 obs.subscribe(i => console.log(`${i} seconds elapsed`));