block scope + direct non-strict eval

[Claus Reinke]

> I bet the "mode" was what got Sam's attention (mine too) in your 
> "classic mode". We are not making hard mode walls or version opt-in. No 
> engine will have a mode enum that must be advanced (implicitly or 
> explicitly) and checked in order to tell what to do in unversioned 
> script. Or so we think!

Btw, how does versioning work across eval? If I have a function body 
with an eval in it, will the body/eval run as in ES5 or as in ES6? Will 
that change if the body or the eval contain ES6 features? Will ES5/ES6
scripts be able to eval ES6/ES5 code?

Claus