[gen_l10n] Create pubspec.yaml in ".dart_tool/flutter_gen" if it does not already exist

[shihaohong]

Fixes dart-lang/build#2835

Tests

I added the following tests:

• A test to ensure that a valid pubspec.yaml file is created
• A test to ensure that existing pubspec.yaml file is not overwritten

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Breaking Change

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[jonahwilliams]

I think this also needs to have an SDK constraint too? I'm not sure if build will prefer the one in the package_config.

[shihaohong]

The build_runner exception no longer appears with a pubspec that does not specify an SDK constraint. Could you clarify what you mean by build preferring the one in package_config over providing one here?

[jonahwilliams]

If it doesn't happen at all, then I would leave this as minimal as possible and with a comment what generated it

[jonahwilliams]

LGTM with question

[kuhnroyal]

This change looks small, can this be picked into a 1.22 hotfix?

[shihaohong]

Since there's a workaround for this and it's not a breaking regression, I don't think it fits into the description of a hotfix: https://github.com/flutter/flutter/wiki/Hotfixes-to-the-Stable-Channel