Start allowing mixin application extensions.

tests/language/mixin_naming_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class S { }
 class M1 { }
 class M2 { }
 class M3 { }
 
 typedef C = S with M1, M2, M3;
+class D extends S with M1, M2, M3 { }
 
 class S_M1 { }
 class S_M1_M2 { }
 
 main() {
   var c = new C();
   Expect.isTrue(c is C);
+  Expect.isFalse(c is D);
   Expect.isTrue(c is S);
   Expect.isFalse(c is S_M1);
   Expect.isFalse(c is S_M1_M2);
 
+  var d = new D();
+  Expect.isFalse(d is C);
+  Expect.isTrue(d is D);
+  Expect.isTrue(d is S);
+  Expect.isFalse(d is S_M1);
+  Expect.isFalse(d is S_M1_M2);
+
   var sm = new S_M1();
   Expect.isFalse(sm is C);
+  Expect.isFalse(sm is D);
   Expect.isFalse(sm is S);
   Expect.isTrue(sm is S_M1);
   Expect.isFalse(sm is S_M1_M2);
 
   var smm = new S_M1_M2();
   Expect.isFalse(smm is C);
+  Expect.isFalse(smm is D);
   Expect.isFalse(smm is S);
   Expect.isFalse(smm is S_M1);
   Expect.isTrue(smm is S_M1_M2);
 }