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

[brianquinlan]

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.

[liamappelbe]

Hmmm. This should happen automatically. Maybe the definition of NSString in the internal headers is doing something weird, and not explicitly extending NSObject?

I suppose we can hard code this behavior.

[brianquinlan]

The declaration for NSString looks like:

@interface NSString : NSObject <NSCopying, NSMutableCopying, NSSecureCoding>

[brianquinlan]

Hmmm... yeah, I see that this usually works but didn't for NSString for some reason.