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
dart-lang/language#4127, the behavior didn't
change, so I didn't notice that these tests needed updating.

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

Bug: dart-lang/language#4127
Change-Id: I59ad1eef7b01ccedcc8fb99e070a05273ac365e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/389781
Reviewed-by: Kallen Tu <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

tests/language/nnbd/flow_analysis/unreachable_via_this_property_test.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=3.1
+
 /// This test verifies that `is` and `==` tests performed on a property get of
 /// `this` do not lead to code being considered unreachable.  (In principle, we
 /// could soundly mark some such code as unreachable, but we have decided not to

tests/language/nnbd/flow_analysis/unreachable_via_variable_property_test.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=3.1
+
 /// This test verifies that `is` and `==` tests performed on a property get of a
 /// variable do not lead to code being considered unreachable.  (In principle,
 /// we could soundly mark some such code as unreachable, but we have decided not