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

headless/index.js

@@ -146,7 +146,7 @@ async function exitActions(
   store: Store,
 ): Promise<void> {
   const {metrics, exit} = userArguments;
-  for (var exitAction of exitClosures) {
+  for (let exitAction of exitClosures) {
     try {
       const action = await exitAction(userArguments, store);
       if (action.exit) {