Add flow types in getEventModifierState (facebook#13909)

Author: armujahid

packages/react-dom/src/events/getEventModifierState.js

@@ -3,13 +3,17 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 /**
  * Translation from modifier key to the associated property in the event.
  * @see http://www.w3.org/TR/DOM-Level-3-Events/#keys-Modifiers
  */
 
+import type {AnyNativeEvent} from 'events/PluginModuleType';
+
 const modifierKeyToProp = {
   Alt: 'altKey',
   Control: 'ctrlKey',
@@ -20,7 +24,7 @@ const modifierKeyToProp = {
 // IE8 does not implement getModifierState so we simply map it to the only
 // modifier keys exposed by the event itself, does not support Lock-keys.
 // Currently, all major browsers except Chrome seems to support Lock-keys.
-function modifierStateGetter(keyArg) {
+function modifierStateGetter(keyArg: string): boolean {
   const syntheticEvent = this;
   const nativeEvent = syntheticEvent.nativeEvent;
   if (nativeEvent.getModifierState) {
@@ -30,7 +34,9 @@ function modifierStateGetter(keyArg) {
   return keyProp ? !!nativeEvent[keyProp] : false;
 }
 
-function getEventModifierState(nativeEvent) {
+function getEventModifierState(
+  nativeEvent: AnyNativeEvent,
+): (keyArg: string) => boolean {
   return modifierStateGetter;
 }