Add language_2 tests demonstrating bug #30568

tests/language_2/abstract_override_adds_optional_args_concrete_test.dart

Index: tests/language_2/abstract_override_adds_optional_args_concrete_test.dart
diff --git a/tests/language_2/abstract_override_adds_optional_args_concrete_test.dart b/tests/language_2/abstract_override_adds_optional_args_concrete_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..b8232e8c0e92155a168d4f062b71e8c67fe3e11a
--- /dev/null
+++ b/tests/language_2/abstract_override_adds_optional_args_concrete_test.dart
@@ -0,0 +1,21 @@
+// 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());
+}