message: refactor(Observable.if): remove ts hacks from Observable
  BREAKING CHANGE: TypeScript users using `Observable.if` will have to cast `Observable` as any to get to `if`. It is a better idae to just use `iif` directly via `import { iif } from 'rxjs';`
diff: import { Observable } from '../../internal/Observable';
  import { iif } from '../../internal/observable/iif';
  -Observable.if = iif;
  +//tslint:disable-next-line:no-any TypeScript doesn't like `if`
  +(Observable as any).if = iif;
  