Add more experimental features from 3.0 into the experiments package (#3352)

* try this

* Hmmmm... patterns?

* Remove the patterns bits completely

* rebuild

* deleted wrong file

Author: jcollins-g

testing/test_package_experiments/analysis_options.yaml

@@ -2,3 +2,5 @@ analyzer:
   # enable for analysis on test package sources.
   enable-experiment:
     - records
+    - sealed-class
+    - class-modifiers

testing/test_package_experiments/lib/class_modifiers.dart

@@ -0,0 +1,24 @@
+// 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.
+
+library class_modifiers.dart;
+
+class A {}
+base class B {}
+interface class C {}
+final class D {}
+sealed class E {}
+abstract class F {}
+abstract base class G {}
+abstract interface class H {}
+abstract final class I {}
+mixin class J {}
+base mixin class K {}
+abstract mixin class L {}
+abstract base mixin class M {}
+mixin N {}
+base mixin O {}
+interface mixin P {}
+final mixin Q {}
+sealed mixin R {}

testing/test_package_experiments/lib/sealed_classes.dart

@@ -0,0 +1,9 @@
+// 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.
+
+library sealed_classes.dart;
+
+sealed class A {
+
+}