[flip-patterns] Pin to tests/vm/dart/regress_flutter51298_test.dart to 2.19.

In Dart 3.0, bool is treated as an "always-exhaustive" type which means
it's a compile error to not exhaustively switch on it.

Since this is a regression test, I don't want to tweak the test and
risk not covering the regressed behavior, so pinning to 2.19 where this
wasn't a compile error.

Change-Id: I51c63730f6db55ecc52daf20888db8778051d98f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/287668
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Siva Annamalai <[email protected]>
Auto-Submit: Bob Nystrom <[email protected]>

Author: munificent

runtime/tests/vm/dart/regress_flutter51298_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=2.19
+
 // VMOptions=--optimization_counter_threshold=10 --deterministic
 
 // Regression test for https://github.com/flutter/flutter/issues/51298.