Start allowing mixin application extensions.

tests/language/mixin_extends_is_test.dart

Index: tests/language/mixin_extends_is_test.dart
diff --git a/tests/language/mixin_is_test.dart b/tests/language/mixin_extends_is_test.dart
similarity index 89%
copy from tests/language/mixin_is_test.dart
copy to tests/language/mixin_extends_is_test.dart
index 2d61ab903b3388f9cd1f8b313a02de1a5485d224..4f08849934ac733c4fc3b272cafa1dc7a3483eda 100644
--- a/tests/language/mixin_is_test.dart
+++ b/tests/language/mixin_extends_is_test.dart
@@ -6,14 +6,14 @@ class S { }
 class M1 { }
 class M2 { }
 
-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 { }
 
-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_ { }
 
 main() {