Disallow var

Summary: We're ES6 and `var`s scoping rules are weird. Let's block this.

Reviewed By: jknoxville

Differential Revision: D16131290

fbshipit-source-id: ba67d16bb8a185a4bb59a657a97b00230dbacafe

src/dispatcher/desktopDevice.js

@@ -12,7 +12,7 @@ import MacDevice from '../devices/MacDevice';
 import WindowsDevice from '../devices/WindowsDevice';
 
 export default (store: Store, logger: Logger) => {
-  var device;
+  let device;
   if (process.platform === 'darwin') {
     device = new MacDevice();
   } else if (process.platform === 'win32') {