Add initialRouteKey for StackRouter (#3540)

* use initialRouteName as key when initializing StackRouter

* fix null headerLeft

* merge back

* fixed tests

* use config flag

* fixed snapshots

* implemented requested changes

flow/react-navigation.js

@@ -362,6 +362,7 @@ declare module 'react-navigation' {
     initialRouteParams?: NavigationParams,
     paths?: NavigationPathsConfig,
     navigationOptions?: NavigationScreenConfig<*>,
+    initialRouteKey?: string,
   |};
 
   declare export type NavigationStackViewConfig = {|

src/routers/StackRouter.js

@@ -91,11 +91,12 @@ export default (routeConfigs, stackConfig = {}) => {
       ...(action.params || {}),
       ...(initialRouteParams || {}),
     };
+    const { initialRouteKey } = stackConfig;
     route = {
       ...route,
       ...(params ? { params } : {}),
       routeName: initialRouteName,
-      key: action.key || generateKey(),
+      key: action.key || (initialRouteKey || generateKey()),
     };
     return {
       key: 'StackRouterRoot',

src/routers/__tests__/StackRouter-test.js

@@ -576,6 +576,23 @@ describe('StackRouter', () => {
     expect(state2.routes[1].routes[1].routes[1].routeName).toEqual('Corge');
   });
 
+  test('Navigate to initial screen is possible', () => {
+    const TestRouter = StackRouter(
+      {
+        foo: { screen: () => <div /> },
+        bar: { screen: () => <div /> },
+      },
+      { initialRouteKey: 'foo' }
+    );
+    const initState = TestRouter.getStateForAction(NavigationActions.init());
+    const pushedState = TestRouter.getStateForAction(
+      NavigationActions.navigate({ routeName: 'foo', key: 'foo' }),
+      initState
+    );
+    expect(pushedState.index).toEqual(0);
+    expect(pushedState.routes[0].routeName).toEqual('foo');
+  });
+
   test('Navigate with key is idempotent', () => {
     const TestRouter = StackRouter({
       foo: { screen: () => <div /> },