Dynamicize some mixin type tests

tests/language/regress_18713_test.dart

 // Copyright (c) 2016, 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 T<X> {
   final Type tType = X;
   Type get getTType => X;
 }
 
 class S<Y> {
   final Type sType = Y;
   Type get getSType => Y;
 }
 
 class TS<A, B> = T<A> with S<B>;
 
+@NoInline() @AssumeDynamic()
+dyn(x) => x;
+
 main() {
   var ts = new TS<int, String>();
+
   Expect.equals("String", ts.sType.toString());
   Expect.equals("int", ts.tType.toString());
   Expect.equals("String", ts.getSType.toString());
   Expect.equals("int", ts.getTType.toString());
+
+  Expect.equals("String", dyn(ts).sType.toString());
+  Expect.equals("int", dyn(ts).tType.toString());
+  Expect.equals("String", dyn(ts).getSType.toString());
+  Expect.equals("int", dyn(ts).getTType.toString());
 }