partition's thisArg should be optional (#8895)

rx/rx.coincidence-lite.d.ts

@@ -29,6 +29,6 @@ declare namespace Rx {
 		*    An array of observables. The first triggers when the predicate returns true,
 		*    and the second triggers when the predicate returns false.
 		*/
-		partition(predicate: (value: T, index: number, source: Observable<T>) => boolean, thisArg: any): Observable<T>[];
+		partition(predicate: (value: T, index: number, source: Observable<T>) => boolean, thisArg?: any): Observable<T>[];
 	}
 }