Enhance pubspec guide

[antfitch]

Description of what this PR is changing or adding, and why:

The existing guide was missing some supported fields and listing some packages as fields. The guide has a very nice robust example, but no robust documentation about the fields and packages in the example. To address this, I added a section called Fields and a section called Packages that goes over the Flutter-specific field and packages, and more specifically, the various data structures that can exist for each field.

@Sfshaza if you think this idea works and you like the general design, @andrewkolos, @vashworth, and @reidbaker can you look closely over the technical details? Am I missing any fields or packages? I have no idea how large the package list is and wasn't sure if we should document this here.

Notes:

• Values that are directories seem to be treated differently in some fields. In some fields, a directory can end with /, and in some fields the / will produce an error.
• Shams, if you like this change, I'll create an issue to link to this PR.

Preview:

• https://flutter-docs-prod--pr11833-conditionally-build-assets-4acsj9hk.web.app/tools/pubspec

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
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[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 574a86b):

https://flutter-docs-prod--pr11833-conditionally-build-assets-4acsj9hk.web.app

[sfshaza2]

@antfitch, this is a huge improvement! lgtm

[vashworth]

I don't know how thorough you want this to be, but I found some other obscure settings:

• deferred-components - https://docs.flutter.dev/perf/deferred-components#how-to-set-your-android-project-up-for-deferred-components
• default-flavor - add default-flavor field to flutter pubspec, which will be used as the flavor in flutter build/run if --flavor is not provided flutter#147968

[antfitch]

I don't know how thorough you want this to be, but I found some other obscure settings:

• deferred-components - https://docs.flutter.dev/perf/deferred-components#how-to-set-your-android-project-up-for-deferred-components
• default-flavor - add default-flavor field to flutter pubspec, which will be used as the flavor in flutter build/run if --flavor is not provided flutter#147968

I believe these have both been turned down. I will double check...

• It looks like default-flavor is available, but only for Android. I'll add this here with a note that this is only for Android.
• I thought I saw somewhere that deferred-components shouldn't be used last week, but I can't find that thread today. I'll add this.

And I've added new sections for both of these. ^_^

[vashworth]

• It looks like default-flavor is available, but only for Android

Why do you think it's only for Android? Seems like at least iOS can use it too

[antfitch]

• It looks like default-flavor is available, but only for Android

Why do you think it's only for Android? Seems like at least iOS can use it too

I see references to it not working in flutter/flutter#147968, but @andrewkolos mentioned that it should work. I will go with the team's impression that it does work.

[antfitch]

@reidbaker can you take another look at the deferred-components field?