fix more mixin tests to work in strong mode

tests/language_strong/mixin_field_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 S {
   var foo = "S-foo";
 }
 
 class M1 {
   final bar = "M1-bar";
 }
 
 class M2 {
   var baz = "M2-baz";
 }
 
 class C = S with M1;
 class D = S with M1, M2;
 class E = S with M2, M1;
 
 class F extends E {
   var fez = "F-fez";
 }
 
 main() {
-  var c = new C();
-  var d = new D();
-  var e = new E();
-  var f = new F();
+  dynamic c = new C();
+  dynamic d = new D();
+  dynamic e = new E();
+  dynamic f = new F();
 
   Expect.equals("S-foo", c.foo);
   Expect.equals("S-foo", d.foo);
   Expect.equals("S-foo", e.foo);
   Expect.equals("S-foo", f.foo);
 
   Expect.equals("M1-bar", c.bar);
   Expect.equals("M1-bar", d.bar);
   Expect.equals("M1-bar", e.bar);
   Expect.equals("M1-bar", f.bar);
@@ skipping 69 matching lines @@
   Expect.throws(() => c.fez = 0, (error) => error is NoSuchMethodError);
   Expect.throws(() => d.fez = 0, (error) => error is NoSuchMethodError);
   Expect.throws(() => e.fez = 0, (error) => error is NoSuchMethodError);
 
   f.fez = "F-fez-f";
   Expect.throws(() => c.fez, (error) => error is NoSuchMethodError);
   Expect.throws(() => d.fez, (error) => error is NoSuchMethodError);
   Expect.throws(() => e.fez, (error) => error is NoSuchMethodError);
   Expect.equals("F-fez-f", f.fez);
 }