Convert react-native-github/Libraries to let/const

Reviewed By: sahrens

Differential Revision: D7961763

fbshipit-source-id: ff7dd1d2c7101c8019e0b4f844d8c377b4d13a13

Author: elicwhite

Libraries/BatchedBridge/MessageQueue.js

@@ -249,7 +249,7 @@ class MessageQueue {
       global.nativeFlushQueueImmediate &&
       now - this._lastFlush >= MIN_TIME_BETWEEN_FLUSHES_MS
     ) {
-      var queue = this._queue;
+      const queue = this._queue;
       this._queue = [[], [], [], this._callID];
       this._lastFlush = now;
       global.nativeFlushQueueImmediate(queue);