'let' loses its block scope when using same argument name in self-invoking function inside for-loop

Invalid behavior:

```
for (let i = 1; i < 3; i += 1) {
  (function (i) {
    console.log(i);
  }(i))
}
console.log(i);
 // 1,2,3
```

Valid behavior:

```
for (let i = 1; i < 3; i += 1) {
  (function (a) {
    console.log(a);
  }(i))
}
console.log(i); 
 // 1, 2, i is not defined
```
