Add mixin language tests.

tests/language/mixin_mixin3_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<List<U>>;
+
+typedef B0 = Object with A<Set<bool>>;
+
+typedef B1 = Object with A<Set<int>>;
+
+class C0 extends B0 { }
+
+class C1 extends B1 { }
+
+typedef A2<K, V> = Object with M<Map<K, V>>;
+
+typedef B2<V> = Object with A2<Set<V>, List<V>>;
+
+typedef B3<K, V> = Object with A2<Set<K>, List<V>>;
+
+class C2<T> extends B2<T> { }
+
+class C3<T> extends B3<T, int> { }
+
+class N {
+  q() { return 42; }
+}
+
+typedef O<U> = Object with N;
+
+typedef P<K, V> = Object with O<V>;
+
+class Q<K, V> extends P<K, V> { }
+
+main() {
+  Expect.equals("List<Set<bool>>", new C0().t().toString());
+  Expect.equals("List<Set<int>>", new C1().t().toString());
+  Expect.equals("Map<Set<bool>, List<bool>>", new C2<bool>().t().toString());
+  Expect.equals("Map<Set<bool>, List<int>>", new C3<bool>().t().toString());
+  Expect.equals(42, new Q<bool, int>().q());
+}