[bug] [safari] Causes a let declaration Syntax Error in Safari.

Oddly, this error doesn't happen in Chrome, only in Safari.

#### Input Code

Here's the non-minified code:

https://cdn.rawgit.com/trusktr/3248affba4a01a7ebf15f3cb0e68f18c/raw/b6b8258ced55798113d852c5b457f0df3bd033c5/not-minified.js

#### Actual Output

Here's the minified code:

https://cdn.rawgit.com/trusktr/3248affba4a01a7ebf15f3cb0e68f18c/raw/b6b8258ced55798113d852c5b457f0df3bd033c5/minified.js

#### Expected Output

It should work, not sure what the output should be. Here's the gist containing both files: https://gist.github.com/trusktr/3248affba4a01a7ebf15f3cb0e68f18c

#### Details

The following codepen uses the non-minified version. It works in Safari, you'll see a spinning square:
https://codepen.io/anon/pen/rzQmpK?editors=0010

The following pen is exactly the same, but it is using the minified version:
https://codepen.io/anon/pen/EvOmJM?editors=0010

In Safari, you will see errors with the minified version, that look like this:


If you run both of those pens in Chrome, they work fine. Only the non-minified pen works in Safari.