Alex

Current [`Set` conformance check](https://github.com/google/closure-compiler/blob/6339e96d57bee2217f0251d593f69b75d71320a0/src/com/google/javascript/jscomp/js/es6/set.js#L36) forces polyfill for `Set`, but will keep [original `Array.from`](https://github.com/google/closure-compiler/blob/6339e96d57bee2217f0251d593f69b75d71320a0/src/com/google/javascript/jscomp/js/es6/array/from.js#L20) in Firefox 32-36. These versions use `set["@@iterator"]` instead of `set[Symbol.iterator]` which leads to `Array.from(new Set("a"))` to be...