Allow super calls in mixins (for now behind a --supermixin flag).

tests/language/mixin_super_bound2_test.dart

+// Copyright (c) 2015, 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.
+// VMOptions=--supermixin
+
+import "package:expect/expect.dart";
+
+bool inCheckedMode() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+}
+
+class MS<U, V extends U> { }
+
+class M<U extends V, V> extends MS<V, U> { }
+
+class NS<U extends V, V> { }
+
+class N<U, V extends U> extends NS<V, U> { }
+
+class S<T> { }
+
+class MNA<U, V, W> extends S<List<U>>
+    with M<List<V>, List<U>>, N<List<W>, List<W>> { }
+
+class MNA2<U, V, W> = S<List<U>>
+    with M<List<W>, List<W>>, N<List<U>, List<V>>;
+
+class MNA3<U, V, W> extends S<List<U>>
+    with MNA<U, V, W>, MNA2<List<U>, List<V>, List<W>> { }
+
+class MNA4<U, V, W> = S<List<U>>
+    with MNA<U, V, W>, MNA2<List<U>, List<V>, List<W>>;
+
+main() {
+  new MNA<num, int, bool>();
+  new MNA2<num, int, bool>();
+  new MNA3<num, int, bool>();
+  new MNA4<num, int, bool>();
+  if (inCheckedMode()) {
+    Expect.throws(() => new MNA<int, num, bool>(), (e) => e is TypeError);
+    Expect.throws(() => new MNA2<int, num, bool>(), (e) => e is TypeError);
+    Expect.throws(() => new MNA3<int, num, bool>(), (e) => e is TypeError);
+    Expect.throws(() => new MNA4<int, num, bool>(), (e) => e is TypeError);
+  } else {
+    new MNA<int, num, bool>();
+    new MNA2<int, num, bool>();
+    new MNA3<int, num, bool>();
+    new MNA4<int, num, bool>();
+  }
+}
+