message: fix(concatAll): use higher-order lettable version of concatAll
diff: import { Observable } from '../Observable';
  import { Subscribable } from '../Observable';
  -import { MergeAllOperator } from './mergeAll';
  +import { concatAll as higherOrder } from '../operators/concatAll';
  /* tslint:disable:max-line-length */
  export function concatAll<T>(this: Observable<T>): T;
  @@ -56,5 +56,5 @@ export function concatAll<T, R>(this: Observable<T>): Subscribable<R>;
  * @owner Observable
  */
  export function concatAll<T>(this: Observable<T>): T {
  - return <any>this.lift<any>(new MergeAllOperator<T>(1));
  + return <any>higherOrder()(this);
  }
  