correct flow types

Summary:
The function should have optional arguments, but not nullable types.
Closes https://github.com/facebook/react-native/pull/7580

Differential Revision: D3304994

Pulled By: davidaurelio

fbshipit-source-id: 9918817429126ea16dd3076b61a3493c3a3a8388

Libraries/CustomComponents/Navigator/Navigation/NavigationRouteStack.js

@@ -117,7 +117,7 @@ class RouteStack {
     return this._routeNodes.findIndex(finder, this);
   }
 
-  slice(begin: ?number, end: ?number): RouteStack {
+  slice(begin?: number, end?: number): RouteStack {
     var routeNodes = this._routeNodes.slice(begin, end);
     var index = Math.min(this._index, routeNodes.size - 1);
     return this._update(index, routeNodes);