linter: Fix bug in no_dynamic_casts w.r.t. Object? for-loop variable

I discovered this bug while migrating Flutter to the new lint rule.

Change-Id: I7944adff58299907e50bf0b2af3b992ec25721ff
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/510142
Auto-Submit: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

CHANGELOG.md

@@ -120,7 +120,7 @@ To learn more about the feature, check out the
 
 - A `no_raw_types` lint rule is introduced, which replaces the
   `strict-raw-types` analysis option, offering a more consistent approach.
-- A `no_dynamic_types` lint rule is introduced, which replaces the
+- A `no_dynamic_casts` lint rule is introduced, which replaces the
   `strict-casts` analysis option, offering a more consistent approach.
 - The following lint rules have been determined to be low value, and are
   deprecated: `avoid_public_typedef_functions`, and `one_member_abstracts`.

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

@@ -306,8 +306,8 @@ class _Visitor extends SimpleAstVisitor<void> {
       elementType = iterableType;
     }
     if (elementType is! DynamicType) return;
-    if (targetType is DynamicType) return;
-    if (targetType == _context.typeProvider.objectQuestionType) return;
+    if (loopVarType is DynamicType) return;
+    if (loopVarType == _context.typeProvider.objectQuestionType) return;
 
     _rule.reportAtNode(node.iterable);
   }

pkg/linter/test/rules/no_dynamic_casts_test.dart

@@ -121,6 +121,14 @@ void f(List<dynamic> list) {
 ''');
   }
 
+  test_forEach_variable_objectQuestionTarget() async {
+    await assertNoDiagnostics(r'''
+void f(List<dynamic> list) {
+  for (Object? x in list) {}
+}
+''');
+  }
+
   test_listLiteral() async {
     await assertDiagnosticsFromMarkdown(r'''
 void f(dynamic a) {