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

tests/language/mixin_override_regression_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 C0 {
   int m1() => 5;
   int m2() => m1();
 }
 
-typedef C1 = Object with C0;
+class C1 = Object with C0;
 
 class D {
   int m1() => 7;
 }
 
 class E0 extends C0 with D {}
 class E1 extends C1 with D {}
 
 main() {
   Expect.equals(7, new E0().m2());
   Expect.equals(7, new E1().m2());
 }