Handle private named parameters in "Extract Widget".

When creating the constructor parameters for the widget, if the field
is private, prefer to use a private named parameter unless it collides
with another public field.

Bug: https://github.com/dart-lang/sdk/issues/61644
Change-Id: I2776e4a350f5034e33bd80f4686e4164e8b0bda8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/479740
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/lib/src/services/refactoring/legacy/extract_widget.dart

@@ -2,6 +2,7 @@
 // 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.
 
+import 'package:_fe_analyzer_shared/src/scanner/token_impl.dart';
 import 'package:analysis_server/src/protocol_server.dart' hide Element;
 import 'package:analysis_server/src/services/correction/status.dart';
 import 'package:analysis_server/src/services/refactoring/legacy/naming_conventions.dart';
@@ -347,18 +348,24 @@ class ExtractWidgetRefactoringImpl extends RefactoringImpl
       }
     }
 
-    // Give each private parameter a public name for the constructor.
+    // Give each private parameter a non-colliding public name if needed.
     for (var parameter in _parameters) {
       var name = parameter.name;
       if (name.startsWith('_')) {
+        // If the library supports private named parameters, we only need to
+        // rename if the public name collides.
         var baseName = name.substring(1);
-        for (var i = 1; ; i++) {
-          name = i == 1 ? baseName : '$baseName$i';
-          if (usedNames.add(name)) {
-            break;
+        if (!_featureSet.isEnabled(Feature.private_named_parameters) ||
+            !usedNames.add(baseName)) {
+          for (var i = 1; ; i++) {
+            name = i == 1 ? baseName : '$baseName$i';
+            if (usedNames.add(name)) {
+              break;
+            }
           }
         }
       }
+
       parameter.constructorName = name;
     }
 
@@ -589,7 +596,10 @@ class ExtractWidgetRefactoringImpl extends RefactoringImpl
       if (parameter != _parameters.first) {
         builder.write(', ');
       }
-      builder.write(parameter.constructorName);
+      builder.write(
+        correspondingPublicName(parameter.constructorName) ??
+            parameter.constructorName,
+      );
       builder.write(': ');
       builder.write(parameter.name);
     }

pkg/analysis_server/test/services/refactoring/legacy/extract_widget_test.dart

@@ -1034,8 +1034,8 @@ class MyWidget extends StatelessWidget {
 class Test extends StatelessWidget {
   const Test({
     super.key,
-    required String field,
-  }) : _field = field;
+    required this._field,
+  });
 
   final String _field;
 
@@ -1098,6 +1098,55 @@ class Test extends StatelessWidget {
 ''');
   }
 
+  Future<void> test_parameters_private_unsupported() async {
+    await indexTestUnit(r'''
+// @dart=3.10
+import 'package:flutter/material.dart';
+
+class MyWidget extends StatelessWidget {
+  final String _field;
+
+  MyWidget(this._field);
+
+  @override
+  Widget build(BuildContext context) {
+    return ^Text(_field);
+  }
+}
+''');
+    _createRefactoring();
+
+    await _assertSuccessfulRefactoring('''
+// @dart=3.10
+import 'package:flutter/material.dart';
+
+class MyWidget extends StatelessWidget {
+  final String _field;
+
+  MyWidget(this._field);
+
+  @override
+  Widget build(BuildContext context) {
+    return Test(field: _field);
+  }
+}
+
+class Test extends StatelessWidget {
+  const Test({
+    super.key,
+    required String field,
+  }) : _field = field;
+
+  final String _field;
+
+  @override
+  Widget build(BuildContext context) {
+    return Text(_field);
+  }
+}
+''');
+  }
+
   Future<void> test_parameters_readField_readLocal() async {
     await indexTestUnit(r'''
 import 'package:flutter/material.dart';