* Allow sending messages from the platform to the framework
* Add ability to send messages from the plugin to the platform.
Also adds [PluginEventChannel] which is the plugin counterpart to
EventChannel.
Fixes https://github.com/flutter/flutter/issues/39981
* fix analyzer errors
* Enhance doc comments
* Remove dead code in test
* WIP on web plugin registry
* WIP on registering plugins
* WIP on web plugin registration
* Only generate `package:flutter_web_plugins` imports if plugins are
defined
* Add parsing test
* Add documentation
* Fix analyzer warnings
* add license headers
* Add tests for package:flutter_web_plugins
* Run `flutter update-packages --force-upgrade`
* Fix analyzer errors
* Fix analyzer error in test
* Update copyright and remove flutter SDK constraints
* Enable tests since engine has rolled
* add flutter_web_plugins tests to bots
* Create an empty .packages file for WebFs test
