Previously, the test made a distinction between null and isA<NotAnnounced> (e.g. compare line 2342 with line 2344). Now, the test cannot differentiate between null and the null that's replacing isA<NotAnnounced> anymore. What test coverage (if any) are we losing because of this?

Also, to verify that the changes in navigator.dart don't introduce any unintended behavior changes: Does this test still pass in its unmodified version (using the test-defined NotAnnounced class) with the modifications to navigator.dart applied? There shouldn't really be a reason to remove the NotAnnounced class from this test, the test could just dispose the instances at the end (or better: the test could create one instance only and dispose that at the end).

I just did that experiment real quick locally and the test is failing, see below. So, the changes in navigator.dart are introducing a change in behavior that the "refactoring" to this test seem to hide.

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following TestFailure was thrown running a test:
Expected: null
  Actual: <Instance of 'NotAnnounced'>

When the exception was thrown, this was the stack:
#4      main.<anonymous closure> (file:///Users/goderbauer/dev/flutter/packages/flutter/test/widgets/navigator_test.dart:2342:5)
<asynchronous suspension>
#5      testWidgets.<anonymous closure>.<anonymous closure> (package:flutter_test/src/widget_tester.dart:183:15)
<asynchronous suspension>
#6      TestWidgetsFlutterBinding._runTestBody (package:flutter_test/src/binding.dart:1021:5)
<asynchronous suspension>
<asynchronous suspension>
(elided one frame from package:stack_trace)

This was caught by the test expectation on the following line:
  file:///Users/goderbauer/dev/flutter/packages/flutter/test/widgets/navigator_test.dart line 2342
The test description was:
  Route announce correctly for first route and last route
════════════════════════════════════════════════════════════════════════════════════════════════════

Thanks. Done. I had to change one line in the test to make it working. This means, yes, the behavior it changed.

So, #144050 is a suggested way.

Sooo... is this changed behavior still correct? Or is this introducing a bug?

It definitely introduced a bug.
I need to be more alerted if tests are updated for a refactoring PR.
Thanks for guidance!