[flutter_plugin_tools] Make unit tests pass on Windows

[stuartmorgan]

The purpose of this PR is to make running all unit tests on Windows pass (vs failing a large portion of the tests as currently happens). This does not mean that the commands actually work when run on Windows, or that Windows support is tested, only that it's possible to actually run the tests themselves. This is prep for actually supporting parts of the tool on Windows in future PRs.

Major changes:

• Make the tests significantly more hermetic:
  • Make almost all tools take a Platform constructor argument that can be used to inject a mock platform to control what OS the command acts like it is running on under test.
  • Add a path Context object to the base command, whose style matches the Platform, and use that almost everywhere instead of the top-level path functions.
  • In cases where Posix behavior is always required (such as parsing git output), explicitly use the posix context object for path functions.
• Start laying the groundwork for actual Windows support:
  • Replace all uses of flutter as a command with a getter that returns flutter or flutter.bat as appropriate.
  • For user messages that include relative paths, use a helper that always uses Posix-style relative paths for consistent output.

Part of flutter/flutter#86113

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[ditman]

Jeez, what a change! Thanks for taking the time to normalize this. Even though this is going to be mostly internal development, is there any way that we can test automatically that people are using "p.posix", "path" and "Platform" consistently through their fixes? Or add some literature somewhere?

This is a pattern that seems "easy" to abandon unless it's enforced somehow.

[stuartmorgan]

is there any way that we can test automatically that people are using "p.posix", "path" and "Platform" consistently through their fixes?

The plan is to start bringing commands online for Windows one at a time, and as I do I'll be adding tests that run in Windows mode. So the enforcement will be in not breaking those tests when making changes.

[ditman]

So the enforcement will be in not breaking those tests when making changes.

Ah, of course, makes sense!