args: Merge allowedHelp into allowed

[DartBot]

This issue was originally filed by @seaneagan

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Currently it looks something like:

  var allowed = {
    'x': 'x help',
    'y': 'y help',
    'z': 'z help',
  }

  parser.addOption('foo', allowed: allowed.keys.toList(), allowedHelp: allowed);

Nicer would be:

  parser.addOption('foo', allowed: {
    'x': 'x help',
    'y': 'y help',
    'z': 'z help',
  });

So allowed: would allow Iterable<String> or Map<String, String>

[floitschG]

Added Area-Pkg, Triaged labels.

[sethladd]

I wonder if the team would be open to a patch for this?

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Removed Priority-Unassigned label.
Added Priority-Low, Pkg-Args labels.

[srawlins]

In order to make this a not breaking change, we could derive allowedHelp from allowed if allowed is a Map, but otherwise still accept an allowed List, and an allowedHelp.

[DartBot]

This issue has been moved to dart-lang/args#25.