Lock "unreachable via this" tests to language version 3.1.

When these tests were written
(https://dart-review.googlesource.com/c/sdk/+/178880), Dart did not
support field promotion. I was getting ready to add "why not promoted"
logic to flow analysis so that if a user tried and failed to promote a
field, and received an assignability error as a result, they would
receive a helpful error message explaining that field promotion was
not supported.

In order to generate this error message, flow analysis would have to
start keeping track of some *counterfactual* promoted types,
indicating what the type of certain expressions *would have been* if
field promotion had been supported. It was important to make sure that
these counterfactual promoted types were only used for error message
generation, and didn't actually change Dart semantics. So I wrote
these tests to help lock down the existing (non-promotion) behavior.

When field promotion was actually implemented in Dart 3.2, these tests
should have been given `@dart=3.1` annotations, since their purpose
was to validate the correct behavior of the implementation in
situations where field promotion *wasn't* enabled. I should have been
prompted to do this by a test failure, because when I enabled field
promotion by default in Dart 3.2, the behavior of the tests should
have changed. However, because of
https://github.com/dart-lang/language/issues/4127, the behavior didn't
change, so I didn't notice that these tests needed updating.

Now, I'm getting ready to fix
https://github.com/dart-lang/language/issues/4127, so in order to
prepare for that, I need to give these tests the proper `@dart=3.1`
annotations.

Bug: https://github.com/dart-lang/language/issues/4127
Change-Id: I59ad1eef7b01ccedcc8fb99e070a05273ac365e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/389781
Reviewed-by: Kallen Tu <kallentu@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>