message: Typed addNavigateHandlers params
diff: import { AppRegistry, NativeEventEmitter, NativeModules } from 'react-native';
  +import { StateNavigator } from 'navigation';
  -var addNavigateHandlers = stateNavigators => {
  +var addNavigateHandlers = (stateNavigator: StateNavigator | StateNavigator[]) => {
  var {NavigationModule} = NativeModules;
  - var stateNavigators = stateNavigators.forEach ? stateNavigators : [stateNavigators];
  + var stateNavigators = isStateNavigator(stateNavigator) ? [stateNavigator] : stateNavigator;
  stateNavigators.forEach((stateNavigator, tab) => {
  stateNavigator.onNavigate(() => {
  var {crumbs, title, history} = stateNavigator.stateContext;
  @@ -12,15 +13,18 @@ var addNavigateHandlers = stateNavigators => {
  }
  });
  });
  - var emitter = new NativeEventEmitter(NavigationModule)
  - emitter.addListener('Navigate', ({crumb, tab}) => {
  + new NativeEventEmitter(NavigationModule).addListener('Navigate', ({crumb, tab}) => {
  var stateNavigator = stateNavigators[tab];
  var distance = stateNavigator.stateContext.crumbs.length - crumb + 1;
  - if (distance > 0) {
  + if (stateNavigator.canNavigateBack(distance)) {
  var url = stateNavigator.getNavigationBackLink(distance);
  stateNavigator.navigateLink(url, undefined, true);
  }
  });
  }
  +function isStateNavigator(stateNavigator: StateNavigator | StateNavigator[]): stateNavigator is StateNavigator {
  + return !!(<StateNavigator> stateNavigator).stateContext;
  +};
  +
  export default addNavigateHandlers;
  