ES6: reinforced the let test

Now, let functionality requires:
* Functional outside "extended mode" and the
"application/javascript;version=1.8" script tag type.
* Temporal dead zone (fixes #131).
* No hoisting out of blocks and for-loop heads.
* For-loops create fresh bindings.

I'm not able to check if EchoJS or IE11 actually pass this reinforced
test, but I'm surprised Traceur doesn't (its 'experimental' output
fails the "qux" check as of today).