Fix unused_element false positive for constructors used via mixin

Closes #62922

GitOrigin-RevId: 72f0d5d
Change-Id: Ib0857acdb71c38cd6c2cef0cb418ca40a8968ee1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/488526
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: rootvector2

pkg/analyzer/lib/src/error/unused_local_elements_verifier.dart

@@ -1233,6 +1233,12 @@ class UsedLocalElements {
       elements.add(element.baseElement);
     } else if (element != null) {
       elements.add(element);
+      var enclosingElement = element.enclosingElement;
+      if (element is ConstructorElementImpl &&
+          enclosingElement is ClassElementImpl &&
+          enclosingElement.isMixinApplication) {
+        addElement(element.superConstructor);
+      }
     }
   }
 

pkg/analyzer/test/src/diagnostics/unused_element_test.dart

@@ -372,6 +372,26 @@ A f() => A._constructor();
 ''');
   }
 
+  test_constructor_isUsed_mixinApplicationRedirect() async {
+    await assertNoErrorsInCode(r'''
+abstract class Foo {
+  factory Foo({required String thing}) = _Foo._;
+  Foo._({required this.thing});
+
+  final String thing;
+
+  void bar();
+}
+
+mixin _$Foo on Foo {
+  @override
+  void bar() {}
+}
+
+class _Foo = Foo with _$Foo;
+''');
+  }
+
   test_constructor_notUsed_multiple() async {
     await assertErrorsInCode(
       r'''