ES6: Support "for...of"

Hi,

simply copy/pasting an example from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of

let arr = [ 3, 5, 7 ];
arr.foo = "hello";

for (let i in arr) {
   console.log(i); // logs "0", "1", "2", "foo"
}

for (let i of arr) {
   console.log(i); // logs "3", "5", "7"
}

for...in puts the key in the variable you define in the for statement.
for...of puts the value in the variable you define in the for statement.

Having support for that would simplify some loops. Not greatly and easy to "workaround" but it would still be nice to have.