IE not working with for...of

[christieb7007]

The app is erroring out in IE 11 with SCRIPT5007: Object expected which is coming from Symbol.iterator from the for...of loops added in 7e15905 and elsewhere (jsutils/mapValue..js#L23, etc)

UPDATE: I was able to fix this in my project by adding the core-js symbol as a polyfill. Perhaps the solution is to add that to https://github.com/graphql/graphql-js/tree/master/src/polyfills

[IvanGoncharov]

@christieb7007 Thanks for the detailed report 👍
Fixed in #2099.
I plan to make release today/tomorrow.