Stop reporting unnecessary_const_in_enum_constructor for constructors outside an enum

Closes https://github.com/dart-lang/sdk/issues/63338

Change-Id: I7b613e22df0eb563076d85ac7f46a7c22a4cb2d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/501561
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/linter/lib/src/rules/unnecessary_const_in_enum_constructor.dart

@@ -48,6 +48,7 @@ class _Visitor extends SimpleAstVisitor<void> {
 
   @override
   void visitConstructorDeclaration(ConstructorDeclaration node) {
+    if (node.parent is! BlockEnumBody) return;
     var constKeyword = node.constKeyword;
     if (constKeyword != null) {
       rule.reportAtToken(constKeyword);
@@ -56,6 +57,7 @@ class _Visitor extends SimpleAstVisitor<void> {
 
   @override
   void visitPrimaryConstructorDeclaration(PrimaryConstructorDeclaration node) {
+    if (node.parent is! EnumDeclaration) return;
     var constKeyword = node.constKeyword;
     if (constKeyword != null) {
       rule.reportAtToken(constKeyword);

pkg/linter/test/rules/unnecessary_const_in_enum_constructor_test.dart

@@ -17,7 +17,13 @@ class UnnecessaryConstInEnumConstructorTest extends LintRuleTest {
   @override
   String get lintRule => LintNames.unnecessary_const_in_enum_constructor;
 
-  test_primary() async {
+  test_primary_inClass() async {
+    await assertNoDiagnostics(r'''
+class const C();
+''');
+  }
+
+  test_primary_inEnum() async {
     await assertDiagnosticsFromMarkdown(r'''
 enum [!const!] E(final int i) {
   a(1), b(2);
@@ -25,7 +31,15 @@ enum [!const!] E(final int i) {
 ''');
   }
 
-  test_secondary() async {
+  test_secondary_inClass() async {
+    await assertNoDiagnostics(r'''
+class C {
+  const C();
+}
+''');
+  }
+
+  test_secondary_inEnum() async {
     await assertDiagnosticsFromMarkdown(r'''
 enum E {
   a(1), b(2);