fix more mixin tests to work in strong mode

tests/language_strong/mixin_field_test.dart

Index: tests/language_strong/mixin_field_test.dart
diff --git a/tests/language_strong/mixin_field_test.dart b/tests/language_strong/mixin_field_test.dart
index e070ed7ee5c6cac95196b25fe93c8ba33b96a08d..59fb0c8623a594818d43763ca408cdbc7e63ffd2 100644
--- a/tests/language_strong/mixin_field_test.dart
+++ b/tests/language_strong/mixin_field_test.dart
@@ -25,10 +25,10 @@ class F extends E {
 }
 
 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);