[pigeon]: added generation of empty classes in dart

[feduke-nukem]

• Added generation of empty classes in dart
• Removed generating data class in Kotlin if class has no fields

closes 160501

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[tarrinneal]

You'll need to run the generate command dart run tools/generate.dart in the pigeon root folder before I can review this pr.

Thanks for putting this together!

[tarrinneal]

No real issues with the implementation here. Thanks for putting this together.

We will need an integration test for this though, you should be able to add it to the already existing 'event channel handles extended sealed classes' test in packages/pigeon/platform_tests/shared_test_plugin_code/lib/integration_tests.dart

The new version needs to be added to the pubspec and generator_tools.dart files as well. I assume those changes got lost during a merge from main.

We should land this one before your other pr to avoid having to update it again.

[tarrinneal]

This should be it's own version update 22.7.3

I think it should read Adds support for empty data classes
This shouldn't be added to dart if it isn't supported on all platforms also, so no [dart] needed

[feduke-nukem]

Done

Also added generating @Suppress("UNUSED_PARAMETER") for fromList() method of empty classes (Kotlin) as tests were failing due to linter

However I am facing issues at some CI steps:

Infra Failure: Step('Run package tests.reset XCode') (canceled) (retcode: None)
Task timed out.

UPD: It worked