Do not share type arguments of mixin types, but clone them to avoid finalization

tests/language/mixin_regress_13688_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 ComparableMixin<E> {
+  e() {
+    return E;
+  }
+}
+
+class KUID extends Object with ComparableMixin<KUID> { }
+
+main() {
+  var kuid = new KUID();
+  Expect.equals(kuid.runtimeType.toString(), kuid.e().toString());
+}