Add nullable annotation for boolean field

[zolotyh]

Now, build_cli can't generate code which matched this:

// file.dart
import 'package:args/args.dart';

void main(Iterable<String> args) {
  final parser = new ArgParser()..addFlag('test', defaultsTo: null, negatable: true);
  final results = parser.parse(args);
  print(results['test']);
}

dart file.dart
// null

I am trying to fix this.

[zolotyh]

@kevmoo I've fixed the particular case. I can do it for other field types or if you show me another way I can do it as you show me.

[kevmoo]

@zolotyh – I tweaked some things here. You'll want to sync down master locally.

Please add a test here – at least modify one of the existing examples in the test directory to use this feature.

[zolotyh]

@kevmoo Thank you for your answer. I've done it. If you have any additional requirements just let me know

[kevmoo]

I was tempted to say we should add some special nullDefault sentinel or something. But this is cleaner.

Thanks!

[zolotyh]

@kevmoo Thank you for approving a request. Do you plan to publish a new version to pub?

[kevmoo]

Working on that now!

…

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