* 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
* Clean up some flutter_tools tests
* Remove arbitrary retry that happens even for fundamental errors, and generally clean up _DevFSHttpWriter.
* Update dependencies (requires fixes; see next commit)
* Fixes for new dependencies.
