extra tests for `Map`, see data-es6.js

Sorry, the compatibility data i have is only for Chrome, Firefox, IE 11.

but it is a little hard to add `for..of`, as it's not ES5 syntax;
and the "iterator protocol" was updated for `Map#keys` only in FireFox 27, seems ( https://bugzilla.mozilla.org/show_bug.cgi?id=881226 )...