Update test file with dot-shorthands to use dart 3.10

[srujzs]

The minimum allowed version of this flag has been updated to 3.10.

[srujzs]

Actually, maybe we can do what @stereotype441 says about not specifying the language version instead.

I tried that (with tls_test.dart) and came across other issues:

[SEVERE] Attempting to enable experiments `[dot-shorthands]`, but the current SDK
    [SEVERE] language version does not match your `analyzer` package language version:
    [SEVERE] 
    [SEVERE] Analyzer language version: 3.9.0
    [SEVERE] SDK language version: 3.10.0
    [SEVERE] 
    [SEVERE] In order to use experiments you may need to upgrade or downgrade your
    [SEVERE] `analyzer` package dependency such that its language version matches that of
    [SEVERE] your current SDK, see https://github.com/dart-lang/build/issues/2685.
    [SEVERE] 
    [SEVERE] Note that you may or may not have a direct dependency on the `analyzer`
    [SEVERE] package in your `pubspec.yaml`, so you may have to add that. You can see your
    [SEVERE] current version by running `pub deps`.

Adding a direct dependency to analyzer feels like the wrong idea here but not sure.