The new values are not ready for use yet inside Google

* Roll engine to e976be13c51448f89107d082ec81e2b6731671fa

* move away from deprecated constants

This updates AppContext per the recommendations in #15352

Fixes #15352

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

If we fail to bind to IPv4 loopback, try IPv6. Some hosts only support IPv6,
causing the test in vmservice_test.dart to fail, since it couldn't find an
available port.

1. Add `PortScanner` abstraction so that we don't do actual port scanning
   in tests.
2. Don't change the real `cwd` of the isolate during tests, as it affects
   all tests, not just the current running test.

Fixes #8761