Suggest fields from declaring parameters in completions

This was preventing suggesting fields from declaring parameters
everywhere.

Closes https://github.com/dart-lang/sdk/issues/63537

Change-Id: I552f85275c6a1aebaa4a50af88d661bc7e3d6492
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/511040
Reviewed-by: Keerti Parthasarathy <keertip@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/lib/src/services/completion/dart/declaration_helper.dart

@@ -1397,7 +1397,9 @@ class DeclarationHelper {
     }
 
     for (var field in element.fields) {
-      if (field.isOriginDeclaration && (!mustBeStatic || field.isStatic)) {
+      if ((field.isOriginDeclaration ||
+              field.isOriginDeclaringFormalParameter) &&
+          (!mustBeStatic || field.isStatic)) {
         _suggestField(
           field: field,
           referencingInterface: referencingInterface,

pkg/analysis_server/test/services/completion/dart/declaration/declaring_parameter_test.dart

@@ -0,0 +1,69 @@
+// 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.
+
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../../../../client/completion_driver_test.dart';
+
+void main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(DeclaringParameterTest);
+  });
+}
+
+@reflectiveTest
+class DeclaringParameterTest extends AbstractCompletionDriverTest {
+  @override
+  bool get includeKeywords => false;
+
+  Future<void> test_insideFunction() async {
+    await computeSuggestions('''
+class C(var int f1) {
+}
+
+void f(C c) {
+  print(c.^);
+}
+''');
+    assertResponse(r'''
+suggestions
+  f1
+    kind: field
+''');
+  }
+
+  Future<void> test_insideMember_sameClass() async {
+    await computeSuggestions('''
+class C(var int f1) {
+  void m() {
+    print(^);
+  }
+}
+''');
+    assertResponse(r'''
+suggestions
+  f1
+    kind: field
+''');
+  }
+
+  Future<void> test_insideMember_subclass() async {
+    await computeSuggestions('''
+class A(var int f1) {}
+
+class B extends A {
+  new() : super(2);
+
+  void m() {
+    print(^);
+  }
+}
+''');
+    assertResponse(r'''
+suggestions
+  f1
+    kind: field
+''');
+  }
+}

pkg/analysis_server/test/services/completion/dart/declaration/test_all.dart

@@ -8,6 +8,7 @@ import 'class_member_test.dart' as class_member;
 import 'class_test.dart' as class_;
 import 'closure_test.dart' as closure;
 import 'constructor_test.dart' as constructor_;
+import 'declaring_parameter_test.dart' as declaring_parameter;
 import 'documentation_test.dart' as documentation;
 import 'enum_test.dart' as enum_;
 import 'extension_member_test.dart' as extension_member;
@@ -32,6 +33,7 @@ void main() {
     class_.main();
     closure.main();
     constructor_.main();
+    declaring_parameter.main();
     documentation.main();
     enum_.main();
     extension_member.main();