Change "typedef" to "class" in core library and related tests.

tests/language/mixin_illegal_super_use_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.
 
 import "package:expect/expect.dart";
 
 class M {
 }
 
 class P0 {
@@ skipping 39 matching lines @@
   }
 }
 
 class P2 {
   baz() {
     super.toString();   /// 11: compile-time error
     return 99;
   }
 }
 
-typedef C = Object with M;
-typedef D = Object with P0;
-typedef E = Object with M, P1;
-typedef F = Object with P2, M;
+class C = Object with M;
+class D = Object with P0;
+class E = Object with M, P1;
+class F = Object with P2, M;
 
 main() {
   var p1 = new P1();
   var p2 = new P2();
   Expect.equals(87, p1.bar());
   p1.test();
   Expect.equals(99, p2.baz());
 }