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

tests/language/mixin_implements2_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.
 
 // Regression test for named mixin applications with implements clause.
 
 class A {}
 
 class S {}
 class M {}
 
-typedef C = S with M implements A;
+class C = S with M implements A;
 
 void main() {
   new C();
 }