How to use "let" or "async/await" on iOS 8 and 9? (babel config?)

I think the problem is with minimum browser settings for babel… but I don’t know how to change them in Meteor.

Boot the iOS simulator for ios 8 or 9. (or use a real device)
Launch Safari on ios 8 or 9.
In the console debugger type:
let x = 1;
(or add it to your Meteor app source code)

You will get a syntax error. It will work on ios 10 and above. Same with async/await.

Is this a problem with babel config? How do I fix?