Check for mandatory when using option

[mosuem]

Fixes #101

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:args	2.7.0	ready to publish	args-v2.7.0
package:async	2.13.0	already published at pub.dev
package:characters	1.4.0	already published at pub.dev
package:collection	1.20.0-wip	WIP (no publish necessary)
package:convert	3.1.2	already published at pub.dev
package:crypto	3.0.6	already published at pub.dev
package:fixnum	1.2.0-wip	WIP (no publish necessary)
package:lints	6.0.0-wip	WIP (no publish necessary)
package:logging	1.3.0	already published at pub.dev
package:os_detect	2.0.3	already published at pub.dev
package:path	1.9.2-wip	WIP (no publish necessary)
package:platform	3.1.6	already published at pub.dev
package:typed_data	1.4.0	already published at pub.dev

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[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
args	Non-Breaking	2.6.0	2.7.0	2.7.0	✔️

Changelog Entry ✔️

Package	Changed Files

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Coverage ✔️

File	Coverage
pkgs/args/lib/src/arg_results.dart	💚 88 % ⬆️ 1 %

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API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

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