Test field promotability for fields "passed through" extension types via "implements".

Change-Id: Ic5bbb5eb12259d076b697bce7624cde181d7beb6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/332484
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Erik Ernst <[email protected]>

Author: stereotype441

tests/language/extension_type/field_promotion_implements_test.dart

@@ -0,0 +1,64 @@
+// 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.
+
+// Verifies that when an extension type declaration includes an "implements"
+// clause, and an attempt is made to promote a property of the underlying
+// representation type, the promotability is inherited from the underlying
+// representation type member.
+
+// SharedOptions=--enable-experiment=inline-class
+
+import '../static_type_helper.dart';
+
+class C {
+  final int? _promotable = 0;
+  int? _notPromotable = 0;
+}
+
+extension type E(C c) implements C {
+  void viaImplicitThis() {
+    if (_promotable != null) {
+      _promotable.expectStaticType<Exactly<int>>();
+    }
+    if (_notPromotable != null) {
+      _notPromotable.expectStaticType<Exactly<int?>>();
+    }
+  }
+
+  void viaExplicitThis() {
+    if (this._promotable != null) {
+      this._promotable.expectStaticType<Exactly<int>>();
+    }
+    if (this._notPromotable != null) {
+      this._notPromotable.expectStaticType<Exactly<int?>>();
+    }
+  }
+}
+
+void viaGeneralPropertyAccess(E e) {
+  if ((e)._promotable != null) {
+    (e)._promotable.expectStaticType<Exactly<int>>();
+  }
+  if ((e)._notPromotable != null) {
+    (e)._notPromotable.expectStaticType<Exactly<int?>>();
+  }
+}
+
+void viaPrefixedIdentifier(E e) {
+  // Note: the analyzer has a special representation for property accesses of
+  // the form `IDENTIFIER.IDENTIFIER`, so we test this form separately.
+  if (e._promotable != null) {
+    e._promotable.expectStaticType<Exactly<int>>();
+  }
+  if (e._notPromotable != null) {
+    e._notPromotable.expectStaticType<Exactly<int?>>();
+  }
+}
+
+main() {
+  E(C()).viaImplicitThis();
+  E(C()).viaExplicitThis();
+  viaGeneralPropertyAccess(E(C()));
+  viaPrefixedIdentifier(E(C()));
+}