This PR makes the static API of the `BackgroundIsolateBinaryMessenger` consistent between its web and IO implementations.
This way, multi-platform applications will compile for the web when using the `ensureInitialized` method (even though it'll throw at runtime, like the `instance` getter, because isolates are not supported on the web).
### Issues
* Fixes: https://github.com/flutter/flutter/issues/145260
* Update project.pbxproj files to say Flutter rather than Chromium
Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.
* Update the copyright notice checker to require a standard notice on all files
* Update copyrights on Dart files. (This was a mechanical commit.)
* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.
Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).
* Clean up the copyrights in non-Dart files. (Manual edits.)
Also, make sure templates don't have copyrights.
* Fix some more ORGANIZATIONNAMEs
