diff --git a/tests/language/deferred/inheritance_constraints_lib.dart b/tests/language/deferred/inheritance_constraints_lib.dart
index 38d3e68bb84..23f4ebcfcf1 100644
--- a/tests/language/deferred/inheritance_constraints_lib.dart
+++ b/tests/language/deferred/inheritance_constraints_lib.dart
@@ -2,6 +2,11 @@
 // 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.
 
+// TODO(https://github.com/dart-lang/sdk/issues/51557): Decide if the mixins
+// being applied in this test should be "mixin", "mixin class" or the test
+// should be left at 2.19.
+// @dart=2.19
+
 class Foo {}
 
 class Foo2 {}
diff --git a/tests/language/deferred/inheritance_constraints_test.dart b/tests/language/deferred/inheritance_constraints_test.dart
index 7477a579281..9bce62ab50a 100644
--- a/tests/language/deferred/inheritance_constraints_test.dart
+++ b/tests/language/deferred/inheritance_constraints_test.dart
@@ -2,6 +2,11 @@
 // 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.
 
+// TODO(https://github.com/dart-lang/sdk/issues/51557): Decide if the mixins
+// being applied in this test should be "mixin", "mixin class" or the test
+// should be left at 2.19.
+// @dart=2.19
+
 import "package:expect/expect.dart";
 import "inheritance_constraints_lib.dart" deferred as lib;
 
diff --git a/tests/language/enum/enhanced_enums_subtype_error_test.dart b/tests/language/enum/enhanced_enums_subtype_error_test.dart
index b623d7d2bdb..14b9c789c72 100644
--- a/tests/language/enum/enhanced_enums_subtype_error_test.dart
+++ b/tests/language/enum/enhanced_enums_subtype_error_test.dart
@@ -2,6 +2,11 @@
 // 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.
 
+// TODO(https://github.com/dart-lang/sdk/issues/51557): Decide if the mixins
+// being applied in this test should be "mixin", "mixin class" or the test
+// should be left at 2.19.
+// @dart=2.19
+
 // Test errors required by new enhanced enum syntax.
 
 // Classes which implement `Enum`, but are not `enum` declarations,
diff --git a/tests/language/mixin/illegal_object_test.dart b/tests/language/mixin/illegal_object_test.dart
index ddd27f236f9..1156580bca3 100644
--- a/tests/language/mixin/illegal_object_test.dart
+++ b/tests/language/mixin/illegal_object_test.dart
@@ -2,6 +2,11 @@
 // 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.
 
+// TODO(https://github.com/dart-lang/sdk/issues/51557): Decide if the mixins
+// being applied in this test should be "mixin", "mixin class" or the test
+// should be left at 2.19.
+// @dart=2.19
+
 // Object has a non-trivial constructor and hence cannot be used as mixin.
 
 class S {}
diff --git a/tests/language/nonfunction_type_aliases/usage_object_error_test.dart b/tests/language/nonfunction_type_aliases/usage_object_error_test.dart
index 373466caedd..ba852405b02 100644
--- a/tests/language/nonfunction_type_aliases/usage_object_error_test.dart
+++ b/tests/language/nonfunction_type_aliases/usage_object_error_test.dart
@@ -2,6 +2,11 @@
 // 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.
 
+// TODO(https://github.com/dart-lang/sdk/issues/51557): Decide if the mixins
+// being applied in this test should be "mixin", "mixin class" or the test
+// should be left at 2.19.
+// @dart=2.19
+
 
 // Introduce an aliased type.
 
diff --git a/tests/language/null/inherit_static_errors_test.dart b/tests/language/null/inherit_static_errors_test.dart
index d6b152e82cc..099abb9bf7f 100644
--- a/tests/language/null/inherit_static_errors_test.dart
+++ b/tests/language/null/inherit_static_errors_test.dart
@@ -2,6 +2,11 @@
 // 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.
 
+// TODO(https://github.com/dart-lang/sdk/issues/51557): Decide if the mixins
+// being applied in this test should be "mixin", "mixin class" or the test
+// should be left at 2.19.
+// @dart=2.19
+
 class BadExtends extends Null {}
 //                       ^^^^
 // [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_DISALLOWED_TYPE
diff --git a/tests/lib/typed_data/restricted_types_error_test.dart b/tests/lib/typed_data/restricted_types_error_test.dart
index 02b572183a3..06ca5fb5a66 100644
--- a/tests/lib/typed_data/restricted_types_error_test.dart
+++ b/tests/lib/typed_data/restricted_types_error_test.dart
@@ -2,6 +2,11 @@
 // 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.
 
+// TODO(https://github.com/dart-lang/sdk/issues/51557): Decide if the mixins
+// being applied in this test should be "mixin", "mixin class" or the test
+// should be left at 2.19.
+// @dart=2.19
+
 import 'dart:typed_data';
 
 abstract class CEByteBuffer extends ByteBuffer {}