FormData definition is degraded

FormData definition is updated at #337.

``` typescript
declare var FormData: {
    prototype: FormData;
    new (form?: HTMLFormElement): FormData;
}
```

But it degraded at a62e7d218d861cba32b2aa8203fd20a55a015c43.

``` typescript
declare var FormData: {
    prototype: FormData;
    new(): FormData;
}
```

Please recovery.
