fix more mixin tests to work in strong mode

tests/language_strong/mixin_super_constructor2_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 Base {
-  int i, j;
+  var i, j;
   Base.ctor1(int i, this.j) : this.i = i + 7;
   Base.ctor2(int i, this.j) : this.i = i + 8;
 }
 
 abstract class M {
   get i;
   get j;
   int k = 42;
   foo() => i + j;
 }
@@ skipping 11 matching lines @@
   Expect.equals(42, c1.k);
   Expect.equals(131, c1.l);
   Expect.equals(21, c1.foo());
   C c2 = new C.ctor2();
   Expect.equals(9, c2.i);
   Expect.equals(13, c2.j);
   Expect.equals(42, c2.k);
   Expect.equals(131, c2.l);
   Expect.equals(22, c2.foo());
 }