addRoute method
Adds a route to the routing table.
Whenever this object routes a PointerEvent corresponding to pointer, call route.
Routes added reentrantly within PointerRouter.route will take effect when routing the next event.
Implementation
void addRoute(int pointer, PointerRoute route) {
final LinkedHashSet<PointerRoute> routes = _routeMap.putIfAbsent(pointer, () => LinkedHashSet<PointerRoute>());
assert(!routes.contains(route));
routes.add(route);
}