Allow enabling custom-devices feature on all channels by default

[ardera]

Currently enabling the custom-devices feature will only take effect on master channel:

flutter/packages/flutter_tools/lib/src/features.dart

Lines 192 to 199 in 9d59532

	const Feature flutterCustomDevicesFeature = Feature(
	name: 'Early support for custom device types',
	configSetting: 'enable-custom-devices',
	environmentOverride: 'FLUTTER_CUSTOM_DEVICES',
	master: FeatureChannelSetting(
	available: true,
	),
	);

While I can understand this is to make sure people don't forget it's not a "stable" feature, it's leading to a lot of problems for the users:

The users are often users of custom embedders, like ivi-homescreen, the sony embedder or flutter-pi. Actually toyota seems to be using it quite a lot. In a custom-embedder scenario, you're often using custom engine builds. And because building the flutter engine is a bit resource-intensive, most people are only doing it for stable:

• toyota is only building the engine for stable engine commits: https://github.com/meta-flutter/flutter_embedded/actions/runs/2778835802
• sony is only building the engine for stable: https://github.com/sony/flutter-embedded-linux/releases
• I too, only build for stable: https://github.com/ardera/flutter-engine-binaries-for-arm

So most people actually want to use custom-devices with the stable channel, which the SDK doesn't allow though. This requires weird workarounds, like going into the SDK dir and checking out the stable SDK commit directly, or checking out the remote branch (origin/stable).

Basically everytime I introduce the feature to someone I have to tell them to use the workaround. Toyota has a script lying around that does nothing else but patch the SDK sources to make it possible to enable the feature on stable.

I'd like to propose we change that and allow the feature to be enabled on any channel. If you're worried that people perceive it as a "stable" feature, we can add some messages like "(this feature is in alpha right now!)" when enabling it. Or we can just say it's stable now

[jonahwilliams]

This sounds good to me. I think we generally want to encourage folks to develop against stable anyway, so at this point the restriction may be do doing more harm than good. If @zanderso @christopherfujino are OK with it I think we should just remove the restriction

[zanderso]

@ardera Would you be willing to expand on the documentation here https://github.com/flutter/flutter/wiki/Using-custom-embedders-with-the-Flutter-CLI as appropriate, and transfer to the website? Under the "Platform integration" docs probably makes sense. You could add a new .md file directly under https://github.com/flutter/website/tree/main/src/development/platform-integration.

[ardera]

@zanderso Yes sure, what expansions did you have in mind? Should I explain more how it works "under the hood", i.e. what commands there are, the available replacement values (${appName}, etc)? I guess once #90958 is merged some documentation for that would need to be added as well.

[zanderso]

what commands there are, the available replacement values (${appName}, etc)? I guess once #90958 is merged some documentation for that would need to be added as well.

Yes, exactly.

[jwinarske]

Just enabling custom-devices on beta and stable is not enough to make it work. Changes from #108884 are also required. Otherwise flutter doctor will fail to find devices.

[zanderso]

The necessary changes have been CP'd to the release branch. @ardera Would you like to leave this issue open to track adding the documentation?