diff --git a/pkg/analyzer/test/generated/test_support.dart b/pkg/analyzer/test/generated/test_support.dart
index 189a1642370..312f506edfc 100644
--- a/pkg/analyzer/test/generated/test_support.dart
+++ b/pkg/analyzer/test/generated/test_support.dart
@@ -12,7 +12,7 @@ import 'package:analyzer/source/line_info.dart';
 import 'package:analyzer/source/source.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer_testing/src/analysis_rule/pub_package_resolution.dart';
-import 'package:analyzer_utilities/extensions/string.dart';
+import 'package:analyzer_testing/utilities/extensions/string.dart';
 import 'package:test/test.dart';
 
 /// A diagnostic listener that collects all of the diagnostics passed to it for
diff --git a/pkg/analyzer_testing/api.txt b/pkg/analyzer_testing/api.txt
index f821cf6097f..6dfcf78cee8 100644
--- a/pkg/analyzer_testing/api.txt
+++ b/pkg/analyzer_testing/api.txt
@@ -65,6 +65,13 @@ package:analyzer_testing/src/analysis_rule/pub_package_resolution.dart:
 package:analyzer_testing/utilities/extensions/resource_provider.dart:
   ResourceProviderExtension (extension on ResourceProvider):
     convertPath (method: String Function(String))
+package:analyzer_testing/utilities/extensions/string.dart:
+  StringExtension (extension on String):
+    isCamelCase (getter: bool)
+    isPascalCase (getter: bool)
+    toCamelCase (method: String Function())
+    toPascalCase (method: String Function())
+    toSnakeCase (method: String Function())
 package:analyzer_testing/utilities/utilities.dart:
   analysisOptionsContent (function: String Function({Map<String, Object?> errors, List<String> experiments, List<String> includes, List<String> legacyPlugins, bool propagateLinterExceptions, List<String> rules, bool strictCasts, bool strictInference, bool strictRawTypes, List<String> unignorableNames}))
   pubspecYamlContent (function: String Function({List<String> dependencies, String? name, String? sdkVersion}))
diff --git a/pkg/analyzer_testing/lib/utilities/extensions/string.dart b/pkg/analyzer_testing/lib/utilities/extensions/string.dart
new file mode 100644
index 00000000000..1be252313f1
--- /dev/null
+++ b/pkg/analyzer_testing/lib/utilities/extensions/string.dart
@@ -0,0 +1,94 @@
+// Copyright (c) 2025, 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.
+
+extension StringExtension on String {
+  static final _toSnakeCaseRegExp = RegExp('_?[A-Z]');
+  static final _startsWithLowerCaseRegExp = RegExp('^[a-z]');
+  static final _startsWithUpperCaseRegExp = RegExp('^[A-Z]');
+
+  /// Returns `true` if the string is a `camelCase` string.
+  bool get isCamelCase {
+    if (contains('_')) return false;
+    if (_startsWithLowerCaseRegExp.matchAsPrefix(this) == null) return false;
+    return true;
+  }
+
+  /// Returns `true` if the string is a `PascalCase` string.
+  bool get isPascalCase {
+    if (contains('_')) return false;
+    if (_startsWithUpperCaseRegExp.matchAsPrefix(this) == null) return false;
+    return true;
+  }
+
+  /// Converts `SCREAMING_SNAKE_CASE` or `snake_case` to `camelCase`.
+  String toCamelCase() {
+    var parts = toLowerCase().split('_');
+    var buffer = StringBuffer();
+    var i = 0;
+    // Preserve initial '_'s
+    while (i < parts.length - 1 && parts[i].isEmpty) {
+      buffer.write('_');
+      ++i;
+    }
+    if (i < parts.length) {
+      // Convert first word to lower case
+      buffer.write(parts[i].toLowerCase());
+      ++i;
+      // Convert remaining words to initial upper case
+      while (i < parts.length) {
+        var part = parts[i];
+        if (part.isNotEmpty) {
+          buffer.write(part[0].toUpperCase());
+          buffer.write(part.substring(1));
+        }
+        ++i;
+      }
+    }
+    return buffer.toString();
+  }
+
+  /// Converts `SCREAMING_SNAKE_CASE` or `snake_case` to `PascalCase`.
+  String toPascalCase() {
+    var parts = toLowerCase().split('_');
+    var buffer = StringBuffer();
+    var i = 0;
+    // Preserve initial '_'s
+    while (i < parts.length - 1 && parts[i].isEmpty) {
+      buffer.write('_');
+      ++i;
+    }
+    // Convert words to initial upper case
+    while (i < parts.length) {
+      var part = parts[i];
+      if (part.isNotEmpty) {
+        buffer.write(part[0].toUpperCase());
+        buffer.write(part.substring(1));
+      }
+      ++i;
+    }
+    return buffer.toString();
+  }
+
+  /// Converts `camelCase` or `PascalCase` to `snake_case`
+  String toSnakeCase() {
+    var parts = <String>[];
+    var i = 0;
+    var wordStarts = _toSnakeCaseRegExp.allMatches(this);
+    for (var RegExpMatch(:start) in wordStarts) {
+      if (i < start) {
+        parts.add(substring(i, start).toLowerCase());
+        i = start;
+      }
+      if (this[i] == '_' && parts.isNotEmpty) {
+        // Avoid doubling up the `_`. This handles strings that are already in
+        // snake case like `foo_Bar` (which translates to `foo_bar`).
+        i++;
+      }
+    }
+    if (i < length) {
+      parts.add(substring(i).toLowerCase());
+    }
+    return parts.join('_');
+  }
+}
diff --git a/pkg/analyzer_testing/pubspec.yaml b/pkg/analyzer_testing/pubspec.yaml
index e8bd20bfe2d..e14ef04af04 100644
--- a/pkg/analyzer_testing/pubspec.yaml
+++ b/pkg/analyzer_testing/pubspec.yaml
@@ -24,3 +24,4 @@ dev_dependencies:
   analyzer_utilities: any
   args: any
   lints: any
+  test_reflective_loader: any
diff --git a/pkg/analyzer_utilities/test/extensions/string_test.dart b/pkg/analyzer_testing/test/utilities/extensions/string_test.dart
similarity index 97%
rename from pkg/analyzer_utilities/test/extensions/string_test.dart
rename to pkg/analyzer_testing/test/utilities/extensions/string_test.dart
index e027799df1d..85366555397 100644
--- a/pkg/analyzer_utilities/test/extensions/string_test.dart
+++ b/pkg/analyzer_testing/test/utilities/extensions/string_test.dart
@@ -6,7 +6,7 @@
 // compatible with the `non_constant_identifier_names` lint.
 // ignore_for_file: non_constant_identifier_names
 
-import 'package:analyzer_utilities/extensions/string.dart';
+import 'package:analyzer_testing/utilities/extensions/string.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
diff --git a/pkg/analyzer_utilities/lib/analyzer_messages.dart b/pkg/analyzer_utilities/lib/analyzer_messages.dart
index f589abb6df2..1814d72f32f 100644
--- a/pkg/analyzer_utilities/lib/analyzer_messages.dart
+++ b/pkg/analyzer_utilities/lib/analyzer_messages.dart
@@ -9,8 +9,8 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:analyzer_testing/package_root.dart' as pkg_root;
+import 'package:analyzer_testing/utilities/extensions/string.dart';
 import 'package:analyzer_utilities/analyzer_message_constant_style.dart';
-import 'package:analyzer_utilities/extensions/string.dart';
 import 'package:analyzer_utilities/located_error.dart';
 import 'package:analyzer_utilities/messages.dart';
 import 'package:analyzer_utilities/tools.dart';
diff --git a/pkg/analyzer_utilities/lib/extensions/string.dart b/pkg/analyzer_utilities/lib/extensions/string.dart
index 1be252313f1..c325d823539 100644
--- a/pkg/analyzer_utilities/lib/extensions/string.dart
+++ b/pkg/analyzer_utilities/lib/extensions/string.dart
@@ -1,94 +1,3 @@
 // Copyright (c) 2025, 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.
-
-extension StringExtension on String {
-  static final _toSnakeCaseRegExp = RegExp('_?[A-Z]');
-  static final _startsWithLowerCaseRegExp = RegExp('^[a-z]');
-  static final _startsWithUpperCaseRegExp = RegExp('^[A-Z]');
-
-  /// Returns `true` if the string is a `camelCase` string.
-  bool get isCamelCase {
-    if (contains('_')) return false;
-    if (_startsWithLowerCaseRegExp.matchAsPrefix(this) == null) return false;
-    return true;
-  }
-
-  /// Returns `true` if the string is a `PascalCase` string.
-  bool get isPascalCase {
-    if (contains('_')) return false;
-    if (_startsWithUpperCaseRegExp.matchAsPrefix(this) == null) return false;
-    return true;
-  }
-
-  /// Converts `SCREAMING_SNAKE_CASE` or `snake_case` to `camelCase`.
-  String toCamelCase() {
-    var parts = toLowerCase().split('_');
-    var buffer = StringBuffer();
-    var i = 0;
-    // Preserve initial '_'s
-    while (i < parts.length - 1 && parts[i].isEmpty) {
-      buffer.write('_');
-      ++i;
-    }
-    if (i < parts.length) {
-      // Convert first word to lower case
-      buffer.write(parts[i].toLowerCase());
-      ++i;
-      // Convert remaining words to initial upper case
-      while (i < parts.length) {
-        var part = parts[i];
-        if (part.isNotEmpty) {
-          buffer.write(part[0].toUpperCase());
-          buffer.write(part.substring(1));
-        }
-        ++i;
-      }
-    }
-    return buffer.toString();
-  }
-
-  /// Converts `SCREAMING_SNAKE_CASE` or `snake_case` to `PascalCase`.
-  String toPascalCase() {
-    var parts = toLowerCase().split('_');
-    var buffer = StringBuffer();
-    var i = 0;
-    // Preserve initial '_'s
-    while (i < parts.length - 1 && parts[i].isEmpty) {
-      buffer.write('_');
-      ++i;
-    }
-    // Convert words to initial upper case
-    while (i < parts.length) {
-      var part = parts[i];
-      if (part.isNotEmpty) {
-        buffer.write(part[0].toUpperCase());
-        buffer.write(part.substring(1));
-      }
-      ++i;
-    }
-    return buffer.toString();
-  }
-
-  /// Converts `camelCase` or `PascalCase` to `snake_case`
-  String toSnakeCase() {
-    var parts = <String>[];
-    var i = 0;
-    var wordStarts = _toSnakeCaseRegExp.allMatches(this);
-    for (var RegExpMatch(:start) in wordStarts) {
-      if (i < start) {
-        parts.add(substring(i, start).toLowerCase());
-        i = start;
-      }
-      if (this[i] == '_' && parts.isNotEmpty) {
-        // Avoid doubling up the `_`. This handles strings that are already in
-        // snake case like `foo_Bar` (which translates to `foo_bar`).
-        i++;
-      }
-    }
-    if (i < length) {
-      parts.add(substring(i).toLowerCase());
-    }
-    return parts.join('_');
-  }
-}
diff --git a/pkg/analyzer_utilities/lib/messages.dart b/pkg/analyzer_utilities/lib/messages.dart
index 7bde00ea9c5..ce27d116d1c 100644
--- a/pkg/analyzer_utilities/lib/messages.dart
+++ b/pkg/analyzer_utilities/lib/messages.dart
@@ -9,8 +9,8 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:analyzer_testing/package_root.dart' as pkg_root;
+import 'package:analyzer_testing/utilities/extensions/string.dart';
 import 'package:analyzer_utilities/analyzer_messages.dart';
-import 'package:analyzer_utilities/extensions/string.dart';
 import 'package:analyzer_utilities/lint_messages.dart';
 import 'package:analyzer_utilities/located_error.dart';
 import 'package:collection/collection.dart';
diff --git a/pkg/analyzer_utilities/pubspec.yaml b/pkg/analyzer_utilities/pubspec.yaml
index 336134a7a3b..413a38feb8d 100644
--- a/pkg/analyzer_utilities/pubspec.yaml
+++ b/pkg/analyzer_utilities/pubspec.yaml
@@ -23,4 +23,3 @@ dependencies:
 dev_dependencies:
   analyzer_plugin: any
   lints: any
-  test_reflective_loader: any
diff --git a/pkg/front_end/pubspec.yaml b/pkg/front_end/pubspec.yaml
index b2f9cb471b2..6a39baeddee 100644
--- a/pkg/front_end/pubspec.yaml
+++ b/pkg/front_end/pubspec.yaml
@@ -23,6 +23,7 @@ dependencies:
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dev_dependencies:
   analyzer: any
+  analyzer_testing: any
   analyzer_utilities: any
   args: any
   build_integration: any
diff --git a/pkg/front_end/test/messages_suite.dart b/pkg/front_end/test/messages_suite.dart
index 4807ef69e69..f71d0e3f3fe 100644
--- a/pkg/front_end/test/messages_suite.dart
+++ b/pkg/front_end/test/messages_suite.dart
@@ -13,8 +13,8 @@ import 'package:_fe_analyzer_shared/src/messages/diagnostic_message.dart'
         getMessageRelatedInformation;
 import 'package:_fe_analyzer_shared/src/messages/severity.dart'
     show CfeSeverity, severityEnumValues;
-import 'package:analyzer_utilities/extensions/string.dart';
 import 'package:analyzer_utilities/messages.dart';
+import 'package:analyzer_testing/utilities/extensions/string.dart';
 import 'package:front_end/src/api_prototype/compiler_options.dart'
     show CompilerOptions, parseExperimentalArguments, parseExperimentalFlags;
 import 'package:front_end/src/api_prototype/experimental_flags.dart'
diff --git a/pkg/front_end/tool/generate_messages_lib.dart b/pkg/front_end/tool/generate_messages_lib.dart
index 2fdf05ebc80..f67c832b414 100644
--- a/pkg/front_end/tool/generate_messages_lib.dart
+++ b/pkg/front_end/tool/generate_messages_lib.dart
@@ -5,7 +5,7 @@
 /// @docImport 'package:front_end/src/codes/type_labeler.dart';
 library;
 
-import 'package:analyzer_utilities/extensions/string.dart';
+import 'package:analyzer_testing/utilities/extensions/string.dart';
 import 'package:analyzer_utilities/located_error.dart';
 import 'package:analyzer_utilities/messages.dart';