[russellwheatley]

Description

I was going to fix this issue, but realised web never properly cleans up the event listeners in the interop layer. i.e the onDispose() function was never called for onAuthStateChanged & onIdTokenChanged.

Ideally, I would've mocked to see the stream handlers created/removed in FirebaseAuthWeb, but mocking the interop layer with staticInterop was extremely difficult and I opted to create an integration test.

Also userStateChanges isn't a real API so it can't be cleaned up on JS side. The problem is; if you have both userStateChanges & onIdTokenChanged stream handlers and you want to clean them up, we have to check the other stream handler has no listeners which is the purpose of these booleans.

I've also updated the clean up code to match what we already have when initialising auth on web. My local testing shows that stream handlers are cleaned up.

Once this PR is merged, I'll fix the multiple stream handler creation bug for web hot restarts

Related Issues

Replace this paragraph with a list of issues related to this PR from the issue database. Indicate, which of these issues are resolved or fixed by this PR. Note that you'll have to prefix the issue numbers with flutter/flutter#.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

[Lyokone]

Minor comment