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/fb-stubs/Logger.js

@@ -9,7 +9,7 @@ import type {TrackType, Logger} from '../fb-interfaces/Logger';
 import type {Store} from '../reducers/index';
 import ScribeLogger from './ScribeLogger';
 
-var instance: ?StubLogger = null;
+let instance: ?StubLogger = null;
 
 type Args = {
   isHeadless?: boolean,