Remove var in Libraries/vendor/core/merge.js (facebook#22108)

Summary:
Replaces the keywords var with const in Libraries/merge.js

If this PR fixes an issue, type "Fixes #issueNumber" to automatically close the issue when the PR is merged.

Pull requests that expand test coverage are more likely to get reviewed. Add a test case whenever possible!

- [x]  Check npm run flow
- [x] Check npm run flow-check-ios
- [x] Check npm run flow-check-android

[GENERAL] [ENHANCEMENT] [Libraries/core] - remove var
Pull Request resolved: facebook#22108

Differential Revision: D12919282

Pulled By: TheSavior

fbshipit-source-id: e3c8569c47c95ad3d1a061917c90185c2694c78f

Author: yushimatenjin

Libraries/vendor/core/merge.js

@@ -7,7 +7,7 @@
 
 "use strict";
 
-var mergeInto = require('mergeInto');
+const mergeInto = require('mergeInto');
 
 /**
  * Shallow merges two structures into a return value, without mutating either.
@@ -16,8 +16,8 @@ var mergeInto = require('mergeInto');
  * @param {?object} two Optional object with properties to merge from.
  * @return {object} The shallow extension of one by two.
  */
-var merge = function(one, two) {
-  var result = {};
+const merge = function(one, two) {
+  const result = {};
   mergeInto(result, one);
   mergeInto(result, two);
   return result;