ObjC: Overly general type generated for some methods/properties

[brianquinlan]

This declaration:

/*!
    @abstract Returns the HTTP request method of the receiver.
    @result the HTTP request method of the receiver.
*/
@property (nullable, readonly, copy) NSString *HTTPMethod;

Generates this code:

  /// !
  /// @abstract Returns the HTTP request method of the receiver.
  /// @result the HTTP request method of the receiver.
  NSObject? get HTTPMethod {
    final _ret = _lib._objc_msgSend_1(_id, _lib._sel_HTTPMethod1);
    return _ret.address == 0 ? null : NSObject._(_ret, _lib);
  }

Notice how the return type is NSObject? instead of NSString?

[brianquinlan]

Likewise for some methods:

/*!
    @method requestWithURL:
    @abstract Allocates and initializes an NSURLRequest with the given
    URL.
    @discussion Default values are used for cache policy
    (NSURLRequestUseProtocolCachePolicy) and timeout interval (60
    seconds).
    @param URL The URL for the request.
    @result A newly-created and autoreleased NSURLRequest instance.
*/
+ (instancetype)requestWithURL:(NSURL *)URL;

Generates this code:

 /// !
  /// @method requestWithURL:
  /// @abstract Allocates and initializes an NSURLRequest with the given
  /// URL.
  /// @discussion Default values are used for cache policy
  /// (NSURLRequestUseProtocolCachePolicy) and timeout interval (60
  /// seconds).
  /// @param URL The URL for the request.
  /// @result A newly-created and autoreleased NSURLRequest instance.
  static NSMutableURLRequest requestWithURL_(
      NativeCupertinoHttp _lib, NSObject? URL) {
    final _ret = _lib._objc_msgSend_13(_lib._class_NSMutableURLRequest1,
        _lib._sel_requestWithURL_1, URL?._id ?? ffi.nullptr);
    return NSMutableURLRequest._(_ret, _lib);
  }