fix more mixin tests to work in strong mode

tests/language_strong/mixin_method_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 {
   foo() => "S-foo";
   baz() => "S-baz";
 }
@@ skipping 10 matching lines @@
 
 class C = S with M1;
 class D = S with M1, M2;
 class E = S with M2, M1;
 
 class F extends E {
   fez() => "F-fez";
 }
 
 main() {
-  var c = new C();
+  dynamic c = new C();
   Expect.equals("S-foo", c.foo());
   Expect.equals("M1-bar", c.bar());
   Expect.equals("S-baz", c.baz());
   Expect.throws(() => c.fez(), (error) => error is NoSuchMethodError);
 
   var d = new D();
   Expect.equals("S-foo", d.foo());
   Expect.equals("M2-bar", d.bar());
   Expect.equals("M2-baz", d.baz());
   Expect.equals("M2-fez", d.fez());
 
   var e = new E();
   Expect.equals("S-foo", e.foo());
   Expect.equals("M1-bar", e.bar());
   Expect.equals("M2-baz", e.baz());
   Expect.equals("M2-fez", e.fez());
 
   var f = new F();
   Expect.equals("S-foo", f.foo());
   Expect.equals("M1-bar", f.bar());
   Expect.equals("M2-baz", f.baz());
   Expect.equals("F-fez", f.fez());
 }