Flow annotation is incorrectly removed?

```js
require('babel-core').transform('export default function(foo: string, bar: string = baz()) { }', {plugins:['syntax-flow'],passPerPreset:true,presets:['babili']}).code
```
gives me this:
```js
export default function (a: string, b = baz()) {}
```

Shouldn't this be:
```js
export default function (a: string, b: string = baz()) {}
```

/cc @boopathi