Native Array.from and polyfilled Set on Firefox 32-36 without Symbol.iterator support

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 `[]`, although native implementation would return correct result.

I don't know whether
1. Whether it is a concern at all (I think it is even that Firefox 32-36 quite rare they have better feature set than IE11 and probably shouldn't have broken experience)
2. We should expose `@@iterator` in `Set` polyfill
3. We should lookup `@@iterator` in `Array.from`
4. Other options I haven't thought of

I assume that this can affect other collections and methods that expect iterator.