Start allowing mixin application extensions.

tests/language/mixin_extends_method_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 {
   foo() => "S-foo";
   baz() => "S-baz";
 }
 
 class M1 {
   bar() => "M1-bar";
 }
 
 class M2 {
   bar() => "M2-bar";
   baz() => "M2-baz";
   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";
 }
 
 main() {
   var c = new C();
   Expect.equals("S-foo", c.foo());
   Expect.equals("M1-bar", c.bar());
   Expect.equals("S-baz", c.baz());
@@ skipping 10 matching lines @@
   Expect.equals("M1-bar", e.bar());
   Expect.equals("M2-baz", e.baz());
   Expect.equals("M2-fez", e.fez());
 
   var f = new F();
   Expect.equals("S-foo", f.foo());
   Expect.equals("M1-bar", f.bar());
   Expect.equals("M2-baz", f.baz());
   Expect.equals("F-fez", f.fez());
 }