[analysis_server] Remove class modifiers in convert_class_to_enum fix

The class modifiers aren't valid on the resulting enum declaration.

Fixes https://github.com/dart-lang/sdk/issues/61189

Change-Id: Idd8ed4fcfae071d21f25811b2158cfad3178fef5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/509880
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/lib/src/services/correction/dart/convert_class_to_enum.dart

@@ -28,6 +28,7 @@ typedef _Constructors = Map<ConstructorElement, _Constructor>;
 /// the following changes:
 ///
 /// * changes the `class` keyword to `enum`,
+/// * removes any class modifiers,
 /// * removes the `const` keyword from the primary constructor, if there is one,
 /// * converts static fields into enum constant values,
 /// * removes an `int index` field if there is one,
@@ -192,9 +193,12 @@ class _EnumDescription {
   /// Use the [builder] and correction [utils] to apply the change necessary to
   /// convert the class to an enum.
   void applyChanges(DartFileEditBuilder builder, CorrectionUtils utils) {
-    // Replace the keyword.
+    // Replace the class keyword and remove leading class modifiers.
     builder.addSimpleReplacement(
-      range.token(classDeclaration.classKeyword),
+      range.startEnd(
+        classDeclaration.firstTokenAfterCommentAndMetadata,
+        classDeclaration.classKeyword,
+      ),
       'enum',
     );
 

pkg/analysis_server/test/src/services/correction/assist/convert_class_to_enum_test.dart

@@ -900,6 +900,81 @@ enum E {
 ''');
   }
 
+  Future<void> test_withClassModifier_base() async {
+    await resolveTestCode('''
+base class ^_E {
+  static const _E c = _E();
+
+  const _E();
+}
+''');
+    await assertHasAssist('''
+enum _E {
+  c
+}
+''');
+  }
+
+  Future<void> test_withClassModifier_baseMixin() async {
+    await resolveTestCode('''
+base mixin class ^_E {
+  static const _E c = _E();
+
+  const _E();
+}
+''');
+    await assertHasAssist('''
+enum _E {
+  c
+}
+''');
+  }
+
+  Future<void> test_withClassModifier_final() async {
+    await resolveTestCode('''
+final class ^_E {
+  static const _E c = _E();
+
+  const _E();
+}
+''');
+    await assertHasAssist('''
+enum _E {
+  c
+}
+''');
+  }
+
+  Future<void> test_withClassModifier_interface() async {
+    await resolveTestCode('''
+interface class ^_E {
+  static const _E c = _E();
+
+  const _E();
+}
+''');
+    await assertHasAssist('''
+enum _E {
+  c
+}
+''');
+  }
+
+  Future<void> test_withClassModifier_mixin() async {
+    await resolveTestCode('''
+mixin class ^_E {
+  static const _E c = _E();
+
+  const _E();
+}
+''');
+    await assertHasAssist('''
+enum _E {
+  c
+}
+''');
+  }
+
   Future<void> test_withReferencedFactoryConstructor() async {
     await resolveTestCode('''
 class _^E {