fix more mixin tests to work in strong mode

tests/language_strong/mixin_super_bound_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.
 
 import "package:expect/expect.dart";
 
 bool inCheckedMode() {
   try {
-    var i = 42;
-    String s = i;
+    String s = 42 as dynamic;
   } on TypeError catch (e) {
     return true;
   }
   return false;
 }
 
 class M<U extends V, V> {}
 
 class N<U, V extends U> {}
 
 class S<T> {}
 
-class MNA<U, V, W> extends S<List<U>> with M<V, U>, N<List<W>, List<W>> {}
+class MNA<U, V extends U, W> extends S<List<U>>
+    with M<V, U>, N<List<W>, List<W>> {}
 
-class MNA2<U, V, W> = S<List<U>> with M<V, U>, N<List<W>, List<W>>;
+class MNA2<U, V extends U, W> = S<List<U>> with M<V, U>, N<List<W>, List<W>>;
 
 main() {
   new MNA<num, int, bool>();
   new MNA2<num, int, bool>();
   if (inCheckedMode()) {
     // Type parameter U of M must extend type parameter V, but
     // type argument num is not a subtype of int.
     Expect.throws(() => new MNA<int, num, bool>(), (e) => e is TypeError);
     // Type parameter U of M must extend type parameter V, but
     // type argument num is not a subtype of int.
     Expect.throws(() => new MNA2<int, num, bool>(), (e) => e is TypeError);
   } else {
     new MNA<int, num, bool>();
     new MNA2<int, num, bool>();
   }
 }