[flow] typecheck getParam to actually check that key and fallback are valid (#5074)

* Update flow type definition for getParam to actually check key and
fallback are valid

* add changelog entry for `getParam` flow type changes

* Return "any" in `getParam` if params are not defined in the navigation state

* use void in place of any

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+- Update `getParam` flow typings to check `key` and `fallback` arguments, as well as return the correct type automatically.
+
 ## [2.17.0] - [2018-09-25](https://github.com/react-navigation/react-navigation/releases/tag/2.17.0)
 
 ### Changed

flow/react-navigation.js

@@ -533,7 +533,16 @@ declare module 'react-navigation' {
       eventName: string,
       callback: NavigationEventCallback
     ) => NavigationEventSubscription,
-    getParam: (paramName: string, fallback?: any) => any,
+    getParam: <ParamName>(
+      paramName: ParamName,
+      fallback?: $ElementType<$PropertyType<{|
+        ...{| params: {| [ParamName]: void |} |},
+        ...$Exact<S>,
+      |}, 'params'>, ParamName>,
+    ) => $ElementType<$PropertyType<{|
+      ...{| params: {| [ParamName]: void |} |},
+      ...$Exact<S>,
+    |}, 'params'>, ParamName>,
     dangerouslyGetParent: () => NavigationScreenProp<*>,
     isFocused: () => boolean,
     // Shared action creators that exist for all routers