fix InstantiateTransformer export default parsing

In order to detect function export declarations in https://github.com/google/traceur-compiler/blob/master/src/codegeneration/InstantiateModuleTransformer.js#L487, we use the `this.inExport_` value to tell if it is a normal function declaration or an export declaration.

This guard wasn't being disabled with `export default`, meaning the following:

```javascript
export default {
  p: function q() {
  }
}
```

Would think there is an export named `q`. This adds a fix for that behaviour.