Support use-flutter option in groundskeeper workflow

[mosuem]

Adds a use-flutter option (defaulting to false) to groundskeeper.yml.

When use-flutter is true:

• Uses subosito/flutter-action with the specified channel (sdk, defaulting to dev) so that Flutter and Dart are available on PATH for Flutter packages (such as cronet_http or ok_http).

When use-flutter is false (default):

• Uses dart-lang/setup-dart with the specified SDK version / channel (defaulting to dev), keeping setup fast and lightweight for pure Dart packages.

Addresses feedback from dart-lang/http#1959.

[gemini-code-assist]

Code Review

This pull request updates the CHANGELOG.md for the firehose package to document the use of the Flutter dev channel in the groundskeeper workflow to support Flutter packages. There are no review comments, so I have no feedback to provide.

[github-actions]

PR Health

Unused Dependencies ✔️

Package	Status
firehose	✔️ All dependencies utilized correctly.

For details on how to fix these, see dependency_validator.

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
firehose	Non-Breaking	0.13.0	0.13.2-wip	0.13.2-wip	✔️

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API leaks ✔️

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Changelog Entry ✔️

Package	Changed Files

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License Headers ✔️

// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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