message: refactor(iif): use EMPTY
diff: import { Observable, SubscribableOrPromise } from '../Observable';
  import { defer } from './defer';
  -import { empty } from './empty';
  +import { EMPTY } from './empty';
  /**
  * Decides at subscription time which Observable will actually be subscribed.
  @@ -85,9 +85,8 @@ import { empty } from './empty';
  */
  export function iif<T, F>(
  condition: () => boolean,
  - // TODO(benlesh): use EMPTY once #3247 lands
  - trueResult: SubscribableOrPromise<T> = empty(),
  - falseResult: SubscribableOrPromise<F> = empty()
  + trueResult: SubscribableOrPromise<T> = EMPTY,
  + falseResult: SubscribableOrPromise<F> = EMPTY
  ): Observable<T|F> {
  return defer<T|F>(() => condition() ? trueResult : falseResult);
  }
  