args: Merge allowedHelp into allowed

[DartBot]

Issue by seaneagan Originally opened as dart-lang/sdk#15705

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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>

[DartBot]

Comment by floitschG

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Added Area-Pkg, Triaged labels.

[DartBot]

Comment by sethladd

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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.

[DartBot]

Comment by srawlins

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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.

[nex3]

Personally I'm in favor of this.