Args Package: Cannot provide <fill in> value for help in addOption

[DartBot]

This issue was originally filed by @butlermatt

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Frequently, command line helps will do something like:

--out=<output dir> The rest of the help information indented over
                       here which refers to <output dir> argument.

However with the Args package, there is no way to show a 'fill in' option. I can specify allowed arguments and help specific to those allowed arguments. But I can't refer to a fill in value, which can sometimes make the documentation feel kind of awkward.

[sethladd]

Set owner to @munificent.
Added Area-Library, Library-Args, Triaged labels.

[dgrove]

Removed Library-Args label.
Added Package-Args label.

[dgrove]

Removed Area-Library label.
Added area-package label.

[dgrove]

Removed area-package label.
Added Area-Pkg label.

[DartBot]

This comment was originally written by amouravski@google.com

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Removed Package-Args label.
Added Library-Args label.

[munificent]

Removed Type-Defect label.
Added Type-Enhancement label.

[anders-sandholm]

Removed Library-Args label.
Added Pkg-Args label.

[munificent]

Added Started label.

[munificent]

Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/args#15.