Add language_2 tests demonstrating bug #30568

tests/language_2/abstract_override_adds_optional_args_concrete_subclass_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() {}
+}
+
+abstract class B extends A {
+  // If this class were concrete, there would be a problem, since `new
+  // B().foo(42)` would be statically allowed, but would lead to invalid
+  // arguments being passed to A.foo.  But since the class is abstract, there is
+  // no problem.
+  void foo([x]);
+}
+
+class /*@compile-error=unspecified*/ C extends B {
+  // However, there is a problem here because this class is concrete and doesn't
+  // override foo.
+}
+
+void f(B b) {
+  b.foo();
+}
+
+main() {
+  f(new C());
+}