objective_c-v8.0.0

• Use ffigen 19.0.0
• NSArray is now a Dart Iterable and NSMutableArray is now a Dart List.
• NSDictionary and NSMutableDictionary are now Dart Maps.
• NSSet and NSMutableSet are now Dart Sets.
• Add .toNSNumber() extension method to int, double, and num.
• Add DateTime.toNSDate() and NSDate.toDateTime() extension methods.
• Add CFStringRef.toDartString() and CFStringRef.toNSString().
• Add toObjCObject and toDartObject that automatically convert between
  supported Objective C and Dart types.
• Added various interfaces, protocols, categories etc to the built ins, such as
  NSPort and NSTimer.