Matthew Kosarek 69264e4a60
Add the 'windowing' feature flag and use to wrap an implementation for regular windows that always throws (#172478)
## What's new?

- Added the `windowing` feature flag to the project
- Created the internal `_window.dart` API with support for regular
windows
- Implemented a `_WindowingOwnerUnsupported` implementation of the
`WindowingOwner` interface
- Marked all points of the public API as `@internal` with a
corresponding warning documentation
- Threw an `UnsupportedError` where it is appropriate to do so

Note that this PR does **NOT** include a real implementation of
`WindowingOwner` (e.g. for win32 or macOS). That work will be opened as
a follow up.

This work is based off of https://github.com/flutter/flutter/pull/168697
and http://github.com/flutter/flutter/pull/168437, as well as this
design document:
https://docs.flutter.dev/go/multi-window-experimental-apis

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

---------

Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>
2025-08-04 17:54:53 +00:00
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