* Starts using the `--source` flag to compile the dart registrant.

* updated general.shard tests

* Fixed the resident compiler flow

* added integration test

* made the integration test self contained

* renamed generated_main to dart_plugin_registrant

* cleaned up for review

* added task runner for ci

* added bringup and TESTOWNERS

* updated failure message

This reverts commit 5257f029.

* Building shared C source code as part of the native build for platforms Android, iOS, Linux desktop, MacOS desktop, and Windows desktop.
* Sample code doing a synchronous FFI call.
* Sample code doing a long running synchronous FFI call on a helper isolate.
* Use of `package:ffigen` to generate the bindings.

This reverts commit 5fd259be.

Building an application for a desktop platform that transitively included any Dart-based plugins (such as path_provider) broke `flutter test`, because its compilation was overriding the provided main (in this case, the test main) with `generated_main.dart` if it was present. This PR:
- Changes the `flutter test` compilation path to update `generated_main.dart`, so that the tests will work, and will include any registered Dart plugins.
- Makes using `generated_main.dart` during recompile opt-in, to try to reduce the chance of a similar bug happening with other codepaths in the future.

Fixes https://github.com/flutter/flutter/issues/88794

Windows plugins are designed to share implementations between Win32 and
UWP, but not all plugins will support both. This adds a new
'supportedVariants' key to Windows plugins that allows specifying
'win32' and/or 'uwp' (and potentially others in the future in case that
becomes necessary).

Plugins without any supported variants will be assumed to be Win32 for
backward compatibility.

This will allow compiling Windows projects that use Win32-only Windows
plugins (which is currently all of them) in UWP mode. The plugins will
of course throw missing implementation exceptions at runtime, but tehy
won't prevent being able to build as they currently do.

Fixes https://github.com/flutter/flutter/issues/82815

[flutter_tools] generate empty plugin registrants for winuwp and fix local engine for winuwp_ (#79827)

* Revert "Enable dart_plugin_registry_test (#76645)"

This reverts commit 109e0bb9.

* Revert "Apply changes caused by https://github.com/flutter/flutter/pull/76662 (#77093)"

This reverts commit cdca6485.

* Revert "Disable clang format in the plugin registrants (#76662)"

This reverts commit dadbd47d.

* Revert "Disable warnings for the dart plugin registrant (#76561)"

This reverts commit 098ece52.

* Revert "Remove dart_plugin_registry_test timeouts (#76838)"

This reverts commit 1610a274.

* Revert "Implement dartPluginClass support for plugins (#74469)"

This reverts commit b7d48062.

Kick.