Add a test to demonstrate issue #30461

tests/language_2/covariant_subtyping_with_substitution_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.
+import 'package:expect/expect.dart';
+
+typedef F<T>(T x);
+
+class B<T> {
+  void f(F<T> x) {}
+}
+
+abstract class I<U> {
+  void f(U x);
+}
+
+class C<V> extends B<V> implements I<F<V>> {}
+
+void acceptsObject(Object o) {}
+
+void acceptsNum(num n) {}
+
+bool isTypeError(e) => e is TypeError;
+
+void g(I<F<num>> i) {
+  i.f(acceptsObject);
+  // i.f has static type (F<num>)->void, or ((num)->void)->void.  Which means we
+  // are statically allowed to pass acceptsNum to it.  However, if i's runtime
+  // type is C<Object>, then it extends B<Object>, so its f function requires
+  // its argument to be F<Object>.  This means that passing acceptsNum to f
+  // would violate soundness (since acceptsNum has type F<num>, and F<num> is a
+  // supertype of F<Object>).  So we expect a type error here.
+  Expect.throws(() {
+    i.f(acceptsNum);
+  }, isTypeError);
+}
+
+void main() {
+  g(new C<Object>());
+}