ObjC: Class hierarchy should be duplicated in generated Dart code.

Objective-C interfaces are converted to Dart classes that extend _ObjCWrapper e.g.

```
class NSString extends _ObjCWrapper {
```

Instead, they should extend their actual superclass e.g.

```
class NSString extends NSObject {
```

Otherwise `NSString` cannot be passed when `NSObject` is expected.

FYI, `castFrom` can be used as a workaround.