Async function makes all variables constant

**What version of TypeScript are you using?**
2.1.4

**What version of `typescript-eslint-parser` are you using?**
1.0.1

**What code were you trying to parse?**

```
async function test() {
     var foo = null;
     foo = 'hhh';
}

```

**What did you expect to happen?**
I excepted no errors.

**What happened?**
Eslint error 'foo is constant (no-const-assign)'


