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

.eslintrc.js

@@ -21,6 +21,7 @@ module.exports = {
     'no-catch-shadow': 0, // only relevant for IE8 and below
     'no-bitwise': 0, // bitwise operations needed in some places
     'consistent-return': 0,
+    'no-var': 2,
     'max-len': 0, // let's take prettier take care of this
     indent: 0, // let's take prettier take care of this
     'no-console': 0, // we're setting window.console in App.js