fix(flatten): fix type inference hinting

Makes it impossible to call flatten() on a stream of numbers, in TypeScript.

src/index.ts

@@ -1932,12 +1932,12 @@ export class Stream<T> implements InternalListener<T> {
    *
    * @return {Stream}
    */
-  flatten<R>(): T {
+  flatten<R>(this: Stream<Stream<R>>): T {
     const p = this._prod;
     return new Stream<R>(
       p instanceof MapOp && !(p instanceof FilterMapFusion) ?
         new MapFlatten(p as MapOp<any, Stream<R>>) :
-        new Flatten(this as any as Stream<Stream<R>>)
+        new Flatten(this)
     ) as T & Stream<R>;
   }
 

tests/operator/flatten.ts

@@ -34,7 +34,7 @@ describe('Stream.prototype.flatten', () => {
     it('should expand each periodic event with 3 sync events', (done: any) => {
       const source: Stream<Stream<number>> = xs.periodic(100).take(3)
         .map((i: number) => xs.of(1 + i, 2 + i, 3 + i));
-      const stream: Stream<number> = source.flatten();
+      const stream = source.flatten();
       const expected = [1, 2, 3, 2, 3, 4, 3, 4, 5];
 
       stream.addListener({