Dynamicize some mixin type tests

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> { }
 
+@NoInline() @AssumeDynamic()
+dyn(x) => x;
+
 main() {
   var kuid = new KUID();
   Expect.equals(kuid.runtimeType.toString(), kuid.e().toString());
+  Expect.equals(dyn(kuid).runtimeType.toString(), dyn(kuid).e().toString());
 }