ECMA 6 - Implement Let and Const Declarations

[pemattio]

The runtime need to has the visitor for the let and const statements.
For this implementation please check this link related to current supported specification.

Examples of errors

const pi=3.14
const pi=3.14

SyntaxError: redeclaration of const pi

const pi=3.14
pi=3.14

TypeError: invalid assignment to const `pi'

if(true) {let a = 2} a;

ReferenceError: a is not defined

let a=1; if(true) {console.log(a); let a=2}

ReferenceError: can't access lexical declaration `a' before initialization