fix more mixin tests to work in strong mode

tests/language_strong/mixin_extends_field_test.dart

Index: tests/language_strong/mixin_extends_field_test.dart
diff --git a/tests/language_strong/mixin_extends_field_test.dart b/tests/language_strong/mixin_extends_field_test.dart
index 1a80e1b06bab0e6060c0112e284f6132039e5150..e8438e407c87109597018d6e7ec1d2d53d6f7d88 100644
--- a/tests/language_strong/mixin_extends_field_test.dart
+++ b/tests/language_strong/mixin_extends_field_test.dart
@@ -27,10 +27,10 @@ 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);