Part of work on [#101077](https://github.com/flutter/flutter/pull/141194). This is done as a separate PR to avoid a massive diff.
## Context
1. The `FakeCommand` class accepts a list of patterns that's used to match a command given to its `FakeProcessManager`. Since `FakeCommand` can match a list of patterns, not just specifically strings, it can be used to match commands where the exact value of some arguments can't (easily) known ahead of time. For example, a part of the tool may invoke a command with an argument that is the path of a temporarily file that has a randomly-generated basename.
2. The `FakeCommand` class provides on `onRun` parameter, which is a callback that is run when the `FakeProcessManager` runs a command that matches the `FakeCommand` in question.
## Issue
In the event that a `FakeCommand` is constructed using patterns, the test code can't know the exact values used for arguments in the command. This PR proposes changing the type of `onRun` from `VoidCallback?` to `void Function(List<String>)?`. When run, the value `List<String>` parameter will be the full command that the `FakeCommand` matched.
Example:
```dart
FakeCommand(
command: <Pattern>[
artifacts.getArtifactPath(Artifact.engineDartBinary),
'run',
'vector_graphics_compiler',
RegExp(r'--input=/.*\.temp'),
RegExp(r'--output=/.*\.temp'),
],
onRun: (List<String> command) {
final outputPath = (() {
// code to parse `--output` from `command`
})();
testFileSystem.file(outputPath).createSync(recursive: true);
},
)
```
## How we determine the channel name
Historically, we used the current branch's upstream to figure out the current channel name. I have no idea why. I traced it back to https://github.com/flutter/flutter/pull/446/files where @abarth implement this and I reviewed that PR and left no comment on it at the time.
I think this is confusing. You can be on a branch and it tells you that your channel is different. That seems weird.
This PR changes the logic to uses the current branch as the channel name.
## How we display channels
The main reason this PR exists is to add channel descriptions to the `flutter channel` list:
```
ianh@burmese:~/dev/flutter/packages/flutter_tools$ flutter channel
Flutter channels:
master (tip of tree, for contributors)
main (tip of tree, follows master channel)
beta (updated monthly, recommended for experienced users)
stable (updated quarterly, for new users and for production app releases)
* foo_bar
Currently not on an official channel.
ianh@burmese:~/dev/flutter/packages/flutter_tools$
```
## Other changes
I made a few other changes while I was at it:
* If you're not on an official channel, we used to imply `--show-all`, but now we don't, we just show the official channels plus yours. This avoids flooding the screen in the case the user is on a weird channel and just wants to know what channel they're on.
* I made the tool more consistent about how it handles unofficial branches. Now it's always `[user branch]`.
* I slightly adjusted how unknown versions are rendered so it's clearer the version is unknown rather than just having the word "Unknown" floating in the output without context.
* Simplified some of the code.
* Made some of the tests more strict (checking all output rather than just some aspects of it).
* Changed the MockFlutterVersion to implement the FlutterVersion API more strictly.
* I made sure we escape the output to `.metadata` to avoid potential injection bugs (previously we just inlined the version and channel name verbatim with no escaping, which is super sketchy).
* Tweaked the help text for the `downgrade` command to be clearer.
* Removed some misleading text in some error messages.
* Made the `.metadata` generator consistent with the template file.
* Removed some obsolete code to do with the `dev` branch.
## Reviewer notes
I'm worried that there are implications to some of these changes that I am not aware of, so please don't assume I know what I'm doing when reviewing this code. :-)
