Can't use const in a for-of loop

With this `constfail.js`:

``` js
"use strict";

const ok = 3;

for(const x of [1, 2]) {
        console.log(x);
}
```

flow breaks on the `const` in the for-of loop:

```
# cat constfail.js | flow check-contents

-:5:5,9: Unexpected token const

Found 1 error
```
