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

tests/language/mixin_illegal_object_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 C0 extends Object
 with Object                       /// 01: compile-time error
 { }
 
-typedef C1 = Object with Object;  /// 02: compile-time error
+class C1 = Object with Object;  /// 02: compile-time error
 
 main() {
   new C0();
   new C1();                       /// 02: continued
 }