Allow variable declaration in if condition statements

[miniak]

Example:

if (let element = document.getElementyById('foobar')) {
    element.someMethod();
}

This allows to have the variable properly block scoped, making it only available in the conditional block body. It should be possible to use const and let.

[DanielRosenwasser]

As much as I've liked this in other languages, it will probably not get implemented by us unless ECMAScript itself pursues it.