Add mixin language tests.

tests/language/mixin_generic_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 S<T> {
+  s() { return T; }
+}
+
+class M<T> {
+  m() { return T; }
+}
+
+class N<T> {
+  n() { return T; }
+}
+
+class C<U, V> extends S<Map<U, V>> with M<List<U>>, N<Set<V>> { }
+
+main() {
+  var c = new C<int, bool>();
+  Expect.isTrue(c is S<Map<int, bool>>);
+  Expect.equals("Map<int, bool>", c.s().toString());
+  Expect.isTrue(c is M<List<int>>);
+  Expect.equals("List<int>", c.m().toString());
+  Expect.isTrue(c is N<Set<bool>>);
+  Expect.equals("Set<bool>", c.n().toString());
+}