Start allowing mixin application extensions.

tests/language/mixin_extends_method_test.dart

Index: tests/language/mixin_extends_method_test.dart
diff --git a/tests/language/mixin_method_test.dart b/tests/language/mixin_extends_method_test.dart
similarity index 91%
copy from tests/language/mixin_method_test.dart
copy to tests/language/mixin_extends_method_test.dart
index d5643f783b2185e383bbfa86f31dc66c44b722a7..9465ff024c6c7bc65b63a5ac43be94cd5103d8ff 100644
--- a/tests/language/mixin_method_test.dart
+++ b/tests/language/mixin_extends_method_test.dart
@@ -17,9 +17,9 @@ class M2 {
   fez() => "M2-fez";
 }
 
-typedef C = S with M1;
-typedef D = S with M1, M2;
-typedef E = S with M2, M1;
+class C extends S with M1 { }
+class D extends S with M1, M2 { }
+class E extends S with M2, M1 { }
 
 class F extends E {
   fez() => "F-fez";