Add mixin language tests.

tests/language/mixin_mixin4_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 I<T> { }
+
+class J<T> { }
+
+class S<T> { }
+
+class M<T> {
+  t() { return T; }
+}
+
+typedef A<U, V> = Object with M<Map<U, V>> implements I<V>;
+
+typedef C<T, K> = S<T> with A<T, List<K>> implements J<K>;
+
+main() {
+  var c = new C<int, bool>();
+  Expect.equals("Map<int, List<bool>>", c.t().toString());
+  Expect.isTrue(c is I<List<bool>>);
+  Expect.isTrue(c is J<bool>);
+  Expect.isTrue(c is S<int>);
+  Expect.isTrue(c is A<int, List<bool>>);
+  Expect.isTrue(c is M<Map<int, List<bool>>>);
+}