Fixes #3851. Add test for representation variable, fix augmenting_constructors_A09_t02.dart (#3852)

LanguageFeatures/Augmentations/augmenting_constructors_A01_t30.dart

diff --git a/LanguageFeatures/Augmentations/augmenting_constructors_A01_t30.dart b/LanguageFeatures/Augmentations/augmenting_constructors_A01_t30.dart
new file mode 100644
index 0000000..c6bce35
--- /dev/null
+++ b/LanguageFeatures/Augmentations/augmenting_constructors_A01_t30.dart
@@ -0,0 +1,86 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+/// @assertion We say that an augmenting function or constructor's signature
+/// matches an introductory signature if:
+/// - It has the same number of type parameters with the same type parameter
+///   names (same identifiers) and bounds (after type annotation inheritance),
+///   if any (same types, even if they may not be written exactly the same in
+///   case one of the declarations needs to refer to a type using an import
+///   prefix).
+/// - The return type (if not omitted) is the same as the introductory
+///   declaration's return type.
+/// - It has the same number of positional parameters as the introductory
+///   declaration, and the same number of those are optional.
+/// - It has the same set of named parameter names as the introductory
+///   declaration.
+/// - For each corresponding pair of parameters:
+///   - They have the same name. This is trivial for named parameters, but may
+///     fail to hold for positional parameters.
+///   - They have the same type (or the augmenting declaration omits the type).
+///   - They both have the modifier `covariant`, or none of them have it.
+///   - They both have the modifier `required`, or none of them have it.
+/// - For all positional parameters:
+///   - The augmenting function's parameter name is `_`, or
+///   - The augmenting function's parameter name is the same as the name of the
+///     corresponding positional parameter in every preceding declaration that
+///     doesn't have `_` as its name.
+/// ...
+/// In a declaration where a parameter named `n` is declared using `_`, the name
+/// `n` is not in scope and may be resolved elsewhere.
+///
+/// @description Checks that it is a compile-time error if an augmentation
+/// changes the name of an extension type constructor parameter from `_` to any
+/// other name while the name of the representation variable remains `_`.
+/// @author sgrekhov22@gmail.com
+
+// SharedOptions=--enable-experiment=augmentations
+
+extension type ET1(int _) {
+  ET1.name(int _);
+//             ^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+augment extension type ET1 {
+  augment ET1.name(this.id);
+//                 ^^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+extension type ET2.name(int _) {
+  ET2.new(int _);
+//            ^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+augment extension type ET2 {
+  augment ET2(this.id);
+//            ^^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+extension type ET3._(int _) {
+  ET3(int _);
+//        ^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+augment extension type ET3 {
+  augment ET3.new(this.id);
+//                 ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(ET1);
+  print(ET2);
+  print(ET3);
+}

LanguageFeatures/Augmentations/augmenting_constructors_A09_t02.dart

diff --git a/LanguageFeatures/Augmentations/augmenting_constructors_A09_t02.dart b/LanguageFeatures/Augmentations/augmenting_constructors_A09_t02.dart
index 7bfacb2..553d258 100644
--- a/LanguageFeatures/Augmentations/augmenting_constructors_A09_t02.dart
+++ b/LanguageFeatures/Augmentations/augmenting_constructors_A09_t02.dart
@@ -68,7 +68,7 @@
   }
 }
 
-extension type ET3._(int _) {
+extension type ET3._(int id) {
   ET3.new(int _);
 }