Kotlin generator fails when '_' is used as a member

For example:

```
interface MyInterface {
    _: ID
}

type MyInterfaceImpl implements MyInterface {
    _: ID
}
```

This would result in code that fails to compile with this error:

```
Error:
Class 'MyInterfaceImpl' is not abstract and does not implement abstract member:
val _: String?
```

Note: this only happens in the Kotlin generator.