* 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.

The FocusManager constructor was registering global event handlers
on the shared RawKeyboard instance and the global pointer router.
This posed a few problems: (1) there was no way to unregister these
handlers, and (2) instantiating a second FocusManager would overwrite
the existing focus manager's RawKeyboard handler. This was manifesting
in unexpected ways, such as the fact that constructing a second
BuildOwner (for a parallel tree, for instance) was obliterating the
event handler for the main BuildOwner's focus manager, thus messing
with focus.

This change separates those global event registrations into a
dedicated method, registerGlobalHandlers(), and overrides dispose()
to properly unregister those handlers.

This reverts commit 50a8ecb7.

This reverts commit 28db5a0f.