fix more mixin tests to work in strong mode

tests/language_strong/mixin_lib_extends_field_test.dart

Index: tests/language_strong/mixin_lib_extends_field_test.dart
diff --git a/tests/language_strong/mixin_lib_extends_field_test.dart b/tests/language_strong/mixin_lib_extends_field_test.dart
index b54433e1aab4cff3bc83d300652570360b085a03..155ebf3e510621a3aa85b210ccaed83954d04379 100644
--- a/tests/language_strong/mixin_lib_extends_field_test.dart
+++ b/tests/language_strong/mixin_lib_extends_field_test.dart
@@ -22,10 +22,10 @@ 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);