feat(publishLast): add higher-order lettable version of publishLast

Author: benlesh

src/operator/publishLast.ts

@@ -1,13 +1,12 @@
 import { Observable } from '../Observable';
-import { AsyncSubject } from '../AsyncSubject';
-import { multicast } from './multicast';
 import { ConnectableObservable } from '../observable/ConnectableObservable';
-
+import { publishLast as higherOrder } from '../operators';
 /**
  * @return {ConnectableObservable<T>}
  * @method publishLast
  * @owner Observable
  */
 export function publishLast<T>(this: Observable<T>): ConnectableObservable<T> {
-  return multicast.call(this, new AsyncSubject<T>());
+  //TODO(benlesh): correct type-flow through here.
+  return higherOrder()(this) as ConnectableObservable<T>;
 }

src/operators/index.ts

@@ -54,6 +54,7 @@ export { partition } from './partition';
 export { pluck } from './pluck';
 export { publish } from './publish';
 export { publishBehavior } from './publishBehavior';
+export { publishLast } from './publishLast';
 export { race } from './race';
 export { reduce } from './reduce';
 export { refCount } from './refCount';

src/operators/publishLast.ts

@@ -0,0 +1,9 @@
+import { Observable } from '../Observable';
+import { AsyncSubject } from '../AsyncSubject';
+import { multicast } from './multicast';
+import { OperatorFunction } from '../interfaces';
+
+//TODO(benlesh): specify that the second type is actually a ConnectableObservable
+export function publishLast<T>(): OperatorFunction<T, T> {
+  return (source: Observable<T>) => multicast(new AsyncSubject<T>())(source);
+}