[pigeon] Fix enum parameters in Dart test

[stuartmorgan-g]

Fixes the handling of enum parameters in the Dart test class generation for testing host APIs.

Fixes flutter/flutter#105742

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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[stuartmorgan-g]

This generated the same code that I wrote manually as a workaround in flutter/plugins#6635.

[tarrinneal]

Seems to be failing the dart analyze tool. I'm not sure why, as you didn't change anything that would result in those failures.

[stuartmorgan-g]

That failure will be fixed by #2751 (see link there for an explanation of the issue).

[tarrinneal]

lgtm

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auto label is removed for flutter/packages, pr: 2749, due to - The status or check suite repo_checks has failed. Please fix the issues identified (or deflake) before re-applying this label.

[stuartmorgan-g]

Ugh, the workaround we did isn't enough for Pigeon's unusual package structure (dart-lang/pub#3618 (comment)). I think we'll need to flip publishable back to stable again temporarily.

[stuartmorgan-g]

I forgot that publishable was combined into a task with various other things that we ideally want to stay on master. Rather than split it back out, I inserted an ugly workaround into the Cirrus config to pre-fetch all of the sub-packages of pigeon. Hopefully we won't need the hack for long, since it doesn't scale well; if it's not fixed soon we'll probably have to split the task out and move it to stable instead.