Add language_2 tests demonstrating bug #30568

tests/language_2/abstract_override_adds_optional_args_concrete_test.dart

+// Copyright (c) 2017, 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.
+
+class A {
+  void foo() {}
+}
+
+class B extends A {
+  // This class declaration violates soundness, since it allows `new
+  // B().foo(42)`, which would lead to invalid arguments being passed to A.foo.
+  void /*@compile-error=unspecified*/ foo([x]);
+}
+
+void f(B b) {
+  b.foo();
+}
+
+main() {
+  f(new B());
+}