Enable ES6-style block-scoped bindings including let and class in sloppy mode, and modify semantics of const and function to operate according to ES6 in sloppy mode.
Docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/class