Add a test for a generic mixin as mixin.

tests/language/mixin_mixin2_test.dart

+// Copyright (c) 2013, 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";
+
+class M<T> {
+  t() { return T; }
+}
+
+typedef A<U> = Object with M<U>;
+
+typedef B<V> = Object with A<V>;
+
+typedef C<U> = Object with M<List<U>>;
+
+typedef D<V> = Object with C<Set<V>>;
+
+class E extends A<num> { }
+
+class F extends B<String> { }
+
+class G<T> extends C<T> { }
+
+class H<T> extends D<Map<String, T>> { }
+
+main() {
+  Expect.equals("num", new E().t().toString());
+  Expect.equals("String", new F().t().toString());
+  Expect.equals("List<bool>", new G<bool>().t().toString());
+  Expect.equals("List<Set<Map<String, int>>>", new H<int>().t().toString());
+}