Move unnecessary_null_in_if_null_operators tests (#4524)

* Move unnecessary_null_in_if_null_operators tests

* rename

test/rules/all.dart

@@ -146,6 +146,8 @@ import 'unnecessary_new_test.dart' as unnecessary_new;
 import 'unnecessary_null_aware_assignments_test.dart'
     as unnecessary_null_aware_assignments;
 import 'unnecessary_null_checks_test.dart' as unnecessary_null_checks;
+import 'unnecessary_null_in_if_null_operators_test.dart'
+    as unnecessary_null_in_if_null_operators;
 import 'unnecessary_nullable_for_final_variable_declarations_test.dart'
     as unnecessary_nullable_for_final_variable_declarations;
 import 'unnecessary_overrides_test.dart' as unnecessary_overrides;
@@ -285,6 +287,7 @@ void main() {
   unnecessary_new.main();
   unnecessary_null_aware_assignments.main();
   unnecessary_null_checks.main();
+  unnecessary_null_in_if_null_operators.main();
   unnecessary_nullable_for_final_variable_declarations.main();
   unnecessary_overrides.main();
   unnecessary_parenthesis.main();

test/rules/unnecessary_null_in_if_null_operators_test.dart

@@ -0,0 +1,72 @@
+// Copyright (c) 2023, 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 '../rule_test_support.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(UnnecessaryNullInIfNullOperatorsLanguage29Test);
+  });
+}
+
+@reflectiveTest
+class UnnecessaryNullInIfNullOperatorsLanguage29Test extends LintRuleTest {
+  @override
+  String get lintRule => 'unnecessary_null_in_if_null_operators';
+
+  @override
+  String get testPackageLanguageVersion => '2.9';
+
+  test_localVariableDeclaration_noNull() async {
+    await assertNoDiagnostics(r'''
+void f() {
+  var x = 1 ?? 1;
+}
+''');
+  }
+
+  test_localVariableDeclaration_nullOnLeft() async {
+    await assertDiagnostics(r'''
+void f() {
+  var x = null ?? 1;
+}
+''', [
+      lint(21, 9),
+    ]);
+  }
+
+  test_localVariableDeclaration_nullOnRight() async {
+    await assertDiagnostics(r'''
+void f() {
+  var x = 1 ?? null;
+}
+''', [
+      lint(21, 9),
+    ]);
+  }
+
+  test_topLevelVariableDeclaration_noNull() async {
+    await assertNoDiagnostics(r'''
+var x = 1 ?? 1;
+''');
+  }
+
+  test_topLevelVariableDeclaration_nullOnLeft() async {
+    await assertDiagnostics(r'''
+var x = null ?? 1;
+''', [
+      lint(8, 9),
+    ]);
+  }
+
+  test_topLevelVariableDeclaration_nullOnRight() async {
+    await assertDiagnostics(r'''
+var x = 1 ?? null;
+''', [
+      lint(8, 9),
+    ]);
+  }
+}