Type of Object.groupBy and its callback wrong

https://github.com/google/closure-compiler/blob/f2cc20715e86c7dafd6de0f9e06eb2da693903f3/externs/es6.js#L2456

produces multiple signatures that are incorrect. Looks like it was interpreted as

`[ function(T, number=): string ] |symbol|number`

rather than

`function(T, number=): [ string|symbol|number ]`

Start with the referenced externs/es6.js at line 2456 and inspect how Object.groupBy and its callback are declared. Update the signatures so the callback returns a string, symbol, or number and verify that generated or checked Elemental2 types expose that shape without producing multiple incorrect signatures.

Written by the indexing model from the issue text.