[wildcard-variables] Add wildcard import prefix tests.

Bug: https://github.com/dart-lang/sdk/issues/55652
Change-Id: I9f17727bcc96bec9fbb01664244a44710f8f02e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/368061
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

tests/language/wildcard_variables/import/import_error_test.dart

@@ -0,0 +1,35 @@
+// Copyright (c) 2024, 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 prefixes are non-binding. This tests that we can't access the
+// top-level declarations of that imported library.
+
+// SharedOptions=--enable-experiment=wildcard-variables
+
+import 'import_lib.dart' as _;
+
+main() {
+  var value = 'str';
+
+  _.topLevel;
+//^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+  _.C(value);
+//^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+  // Private extensions can't be used.
+  value.bar;
+//^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+  value.fn;
+//^
+// [analyzer] unspecified
+// [cfe] unspecified
+}

tests/language/wildcard_variables/import/import_lib.dart

@@ -0,0 +1,28 @@
+// Copyright (c) 2024, 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.
+
+// SharedOptions=--enable-experiment=wildcard-variables
+
+int topLevel = 1;
+
+class C {
+  final bool bar;
+  final bool fn;
+
+  C(String str)
+      : bar = str.bar,
+        fn = str.fn;
+}
+
+extension StringExtension on String {
+  bool get foo => true;
+}
+
+extension _PrivateStringExtension on String {
+  bool get bar => true;
+}
+
+extension on String {
+  bool get fn => true;
+}

tests/language/wildcard_variables/import/import_test.dart

@@ -0,0 +1,17 @@
+// Copyright (c) 2024, 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 prefixes named `_` are non-binding, but will provide access to the
+// non-private extensions in that library.
+
+// SharedOptions=--enable-experiment=wildcard-variables
+
+import 'package:expect/expect.dart';
+
+import 'import_lib.dart' as _;
+
+main() {
+  var value = 'str';
+  Expect.isTrue(value.foo);
+}