Add custom sdk build options

[schultek]

For #4299

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[Markzipan]

The changes look good!

[github-actions]

PR Health

Changelog Entry ✔️

Package	Changed Files

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[Markzipan]

LGTM!

Feel free to submit after changelog/version updates

[schultek]

Thanks for the approval.

I did one renaming of the option related to allowing unsafe modules, because I had a read through the webdev issue and it seems like this is more related than I initially though. Since I don't want to conflict here with anything discussed there I opted for the renaming prefixed with "unsafe" and also not publicly documenting this flag.

Lmk if the changelog is ok like this, or if I should document the other added flags explicitly.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:build	4.0.3	already published at pub.dev
package:build_config	1.2.0	already published at pub.dev
package:build_daemon	4.1.1	already published at pub.dev
package:build_modules	5.1.4	already published at pub.dev
package:build_runner	2.11.0-wip	WIP (no publish necessary)
package:build_test	3.5.5-wip	WIP (no publish necessary)
package:build_web_compilers	4.4.6	ready to publish	build_web_compilers-v4.4.6
package:scratch_space	1.2.0	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[Markzipan]

Changelog looks fine! 'Non-user-visible' changes don't need to be included there, so your reasoning's sound. Pulling @bkonyi into this as an FYI; they've been following some efforts to allow these kinds of "flutter can use this lib but I can't" kind of situations.

[kevmoo]

Is this documented anywhere? How would one use it?

[schultek]

Currently not, but I'd like to.
Where would be a good place to document this? I think this shouldn't be too visible (so not in Readme) since it's quite a special case.

[kevmoo]

The other build options are in the readme, right?

It's fine to add an "advanced" section or similar. Just don't want this to get lost in the weeds!