What traceur does to your code; compiled via `traceur --block-binding true --module let.js --out let.compiled.js`

let.compiled.js

System.register("let", [], function() {
  "use strict";
  var __moduleName = "let";
  {
    try {
      throw undefined;
    } catch (x) {
      x = 10;
    }
  }
  console.log(x);
  return {};
});
System.get("let" + '');

let.js

{ let x = 10; } console.log(x);