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

tests/language/mixin_issue10216_2_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";
 
-typedef M1 = Object with M0;
-typedef M2 = Object with M1;
+class M1 = Object with M0;
+class M2 = Object with M1;
 
 class M0 {
   foo() => 42;
 }
 
 makeM2() {
   return [new Object(), new M2()].last as M2;
 }
 
 main() {
   Expect.equals(42, makeM2().foo());
 }