Fixes flow errors

Author: davepack

src/TypeDefinition.js

@@ -382,6 +382,8 @@ export type NavigationScreenProp<S, A> = {
     action?: NavigationAction
   ) => boolean,
   setParams: (newParams: NavigationParams) => boolean,
+  setTransitionStart: () => boolean,
+  setTransitionEnd: () => boolean,
 };
 
 export type NavigationNavigatorProps<O, S> = {

src/views/Transitioner.js

@@ -49,7 +49,7 @@ const DefaultTransitionSpec = ({
 
 class Transitioner extends React.Component<*, Props, State> {
   _onLayout: (event: any) => void;
-  _onTransitionEnd: () => void;
+  _onTransitionEnd: (indexHasChanged: boolean) => void;
   _prevTransitionProps: ?NavigationTransitionProps;
   _transitionProps: NavigationTransitionProps;
   _isMounted: boolean;