Starts using the `--source` flag to compile the dart registrant. (#98046)

* 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