ES6 keyword support: const & let

[lateau]

These keywords have same syntax with var and should work well with same token parsing.

((function ()
  'use strict'
  (let a 1)
  (const b 2)
  (var c 3)))

=>

(function() {
    'use strict';
    let a = 1;
    const b = 2;
    var c = 3;
})();

[bowbahdoe]

Is there any reason this hasn't been merged?