[args] Bug: inconsistent newline behavior after Separator Name

[indraneel12]

Consider the following code (can be run in DartPad):

import 'package:args/command_runner.dart' show CommandRunner;

void main() {
  final runner = CommandRunner(
    'mock-command',
    'Mock Command to explain a newline bug in this Package.',
  );
  runner.argParser.addFlag(
    'flag-0',
    help: 'No newline problem after Separator Name when this section exists.',
  );
  runner.argParser.addSeparator('Group 1 (OK)');
  runner.argParser.addFlag('flag-1');
  runner.argParser.addFlag('flag-2');
  runner.argParser.addFlag('flag-3');
  runner.argParser.addSeparator('Group 2 (newline problem)');
  runner.argParser.addFlag('flag-4');
  runner.argParser.addFlag('flag-5', help: 'This section does not work here.');
  runner.argParser.addFlag('flag-6');
  runner.argParser.addSeparator('Group 3 (newline problem)');
  runner.argParser.addFlag('flag-7');
  runner.argParser.addFlag('flag-8');
  runner.argParser.addFlag('flag-9', help: 'This section works at group-end.');
  runner.argParser.addSeparator('Group 4 (OK)');
  runner.argParser.addFlag('flag-10');
  runner.argParser.addFlag('flag-11');
  runner.argParser.addSeparator('Group 5 (newline problem)');
  runner.argParser.addFlag('flag-12');
  print(runner.usage);
}

The output of my Minimal Reproducible Example is self-explanatory:

Mock Command to explain a newline bug in this Package.

Usage: mock-command <command> [arguments]

Global options:
-h, --help            Print this usage information.
    --[no-]flag-0     No newline problem after Separator Name when this section exists.

Group 1 (OK)
    --[no-]flag-1     
    --[no-]flag-2     
    --[no-]flag-3     

Group 2 (newline problem)

    --[no-]flag-4     
    --[no-]flag-5     This section does not work here.
    --[no-]flag-6     

Group 3 (newline problem)

    --[no-]flag-7     
    --[no-]flag-8     
    --[no-]flag-9     This section works at group-end.

Group 4 (OK)
    --[no-]flag-10    
    --[no-]flag-11    

Group 5 (newline problem)

    --[no-]flag-12    

Available commands:
  help   Display help information for mock-command.

Run "mock-command help <command>" for more information about a command.

I was debugging my project, and I guess this problem originates within void _writeLine(int column, String text) from this Package.