Fix missing Alert in react-native.js.flow

Summary: Closes https://github.com/facebook/react-native/pull/5192

Reviewed By: svcscm

Differential Revision: D2815060

Pulled By: androidtrunkagent

fb-gh-sync-id: cbf9058e4e6b36584d8487fc132faa2a5c52bbe0

Libraries/react-native/react-native.js.flow

@@ -67,6 +67,7 @@ var ReactNative = Object.assign(Object.create(require('React')), {
   // APIs
   ActionSheetIOS: require('ActionSheetIOS'),
   AdSupportIOS: require('AdSupportIOS'),
+  Alert: require('Alert'),
   AlertIOS: require('AlertIOS'),
   Animated: require('Animated'),
   AppRegistry: require('AppRegistry'),