Start allowing mixin application extensions.

tests/language/mixin_extends_is_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 { }
 
-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() {
   var c = new C();
   Expect.isTrue(c is C);
   Expect.isFalse(c is D);
   Expect.isFalse(c is E);
   Expect.isFalse(c is F);
   Expect.isTrue(c is S);
   Expect.isTrue(c is M1);
@@ skipping 41 matching lines @@
   Expect.isFalse(e is C_);
   Expect.isFalse(e is D_);
   Expect.isFalse(e is E_);
   Expect.isFalse(e is F_);
 
   Expect.isFalse(f is C_);
   Expect.isFalse(f is D_);
   Expect.isFalse(f is E_);
   Expect.isFalse(f is F_);
 }