Add @flow to BackHandler

Summary:
Add flow to BackHandler
Fixed callsites

Reviewed By: TheSavior

Differential Revision: D8994015

fbshipit-source-id: 2b910f14b4462020a37a33a8bcff67cef07500a3

Author: Timothy Kukulski

Libraries/Utilities/BackHandler.android.js

@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  *
  * @format
+ * @flow
  */
 
 'use strict';
@@ -16,6 +17,7 @@ const DEVICE_BACK_EVENT = 'hardwareBackPress';
 
 type BackPressEventName = $Enum<{
   backPress: string,
+  hardwareBackPress: string,
 }>;
 
 const _backPressSubscriptions = [];
@@ -61,7 +63,7 @@ RCTDeviceEventEmitter.addListener(DEVICE_BACK_EVENT, function() {
  * ```
  */
 const BackHandler = {
-  exitApp: function() {
+  exitApp: function(): void {
     DeviceEventManager.invokeDefaultBackPressHandler();
   },
 
@@ -79,7 +81,7 @@ const BackHandler = {
       _backPressSubscriptions.push(handler);
     }
     return {
-      remove: () => BackHandler.removeEventListener(eventName, handler),
+      remove: (): void => BackHandler.removeEventListener(eventName, handler),
     };
   },
 

Libraries/Utilities/BackHandler.ios.js

@@ -8,6 +8,7 @@
  * On iOS, this just implements a stub.
  *
  * @format
+ * @flow
  */
 
 'use strict';
@@ -17,9 +18,10 @@ const TVEventHandler = require('TVEventHandler');
 
 type BackPressEventName = $Enum<{
   backPress: string,
+  hardwareBackPress: string,
 }>;
 
-function emptyFunction() {}
+function emptyFunction(): void {}
 
 /**
  * Detect hardware button presses for back navigation.
@@ -100,12 +102,12 @@ if (Platform.isTV) {
 } else {
   BackHandler = {
     exitApp: emptyFunction,
-    addEventListener() {
+    addEventListener(_eventName: BackPressEventName, _handler: Function) {
       return {
         remove: emptyFunction,
       };
     },
-    removeEventListener: emptyFunction,
+    removeEventListener(_eventName: BackPressEventName, _handler: Function) {},
   };
 }