fix more mixin tests to work in strong mode

tests/language_strong/mixin_super_constructor_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;
   Base.ctor(int i, this.j) : this.i = i + 7;
 }
 
 abstract class M {
-  get i;
-  get j;
+  int get i;
+  int get j;
   int k = 42;
   foo() => i + j;
 }
 
 class C extends Base with M {
   int l = 131;
   C() : super.ctor(1, 13);
 }
 
 main() {
   C c = new C();
   Expect.equals(8, c.i);
   Expect.equals(13, c.j);
   Expect.equals(42, c.k);
   Expect.equals(131, c.l);
   Expect.equals(21, c.foo());
 }