Fix language tests wrt. warnings.

tests/language/abstract_factory_constructor_test.dart

Index: tests/language/abstract_factory_constructor_test.dart
diff --git a/tests/language/abstract_factory_constructor_test.dart b/tests/language/abstract_factory_constructor_test.dart
index d2e3a6f00219e2106dbf4bee7b58f65644f95c8f..cdd03f94fa4ea04674a3f4ea3879f970aeaa1fc9 100644
--- a/tests/language/abstract_factory_constructor_test.dart
+++ b/tests/language/abstract_factory_constructor_test.dart
@@ -11,13 +11,13 @@ class B extends A1 {
   method() {}
 }
 
-class A1 {
+abstract class A1 {
   A1() {}
   method();  // Abstract.
   factory A1.make() { return new B(); }
 }
 
-class A2 {
+abstract class A2 {
   method();  // Abstract.
   A2.make() {}
 }