Fixes #1899. Language version 2.19 added to legacy mixins tests (#1907)

Add language version 2.19 to legacy mixins tests. Update generator and generated files.

Author: sgrekhov

Language/Classes/definition_t01.dart

@@ -11,6 +11,10 @@
 /// to this syntax do not cause any errors and can be instantiated.
 /// @author msyabro
 
+// 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
 
 abstract class I {}
 abstract class J {}

Language/Classes/definition_t23.dart

@@ -12,6 +12,10 @@
 /// errors and can be instantiated.
 /// @author [email protected]
 
+// 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
 
 @A() abstract class I1<T> = A1 with B1;
 abstract class J1<T> = A1 with B1;

Language/Classes/mixins_t01.dart

@@ -14,6 +14,10 @@
 /// accepted.
 /// @author kaigorodov
 
+// 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 A {
   int a() {return 1;}

Language/Classes/mixins_t02.dart

@@ -13,6 +13,10 @@
 /// @description Checks that mixins without superclass are allowed
 /// @author kaigorodov
 
+// 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 A {
   int a() {return 1;}

Language/Expressions/Lookup/Getter_and_Setter_Lookup/definition_t08.dart

@@ -13,6 +13,11 @@
 /// lookup.
 /// @author ilya
 
+// 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 '../../../../Utils/expect.dart';
 
 class A {

Language/Expressions/Lookup/Method_Lookup/instance_t03.dart

@@ -11,6 +11,11 @@
 /// 
 /// @author [email protected]
 
+// 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 '../../../../Utils/expect.dart';
 
 class A {

Language/Expressions/Lookup/Method_Lookup/superclass_t09.dart

@@ -12,6 +12,11 @@
 /// found during lookup
 /// @author [email protected]
 
+// 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 '../../../../Utils/expect.dart';
 
 class A {

Language/Expressions/Method_Invocation/Ordinary_Invocation/accessible_instance_member_t07.dart

@@ -20,6 +20,11 @@
 /// that extends class Function and doesn't implement method call.
 /// @author [email protected]
 
+// 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 '../../../../Utils/expect.dart';
 
 class A extends Function {

Language/Expressions/Method_Invocation/Ordinary_Invocation/accessible_instance_member_t08.dart

@@ -20,6 +20,11 @@
 /// a class that extends class Function and implements method call.
 /// @author [email protected]
 
+// 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 '../../../../Utils/expect.dart';
 
 class A extends Function {

Language/Expressions/Property_Extraction/Super_Getter_Access_and_Method_Closurization/method_extraction_t03.dart

@@ -13,6 +13,11 @@
 /// property extraction is method that was found in a mixin.
 /// @author [email protected]
 
+// 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 '../../../../Utils/expect.dart';
 
 class A {