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

tests/language/mixin_mixin2_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<T> {
   t() { return T; }
 }
 
-typedef A<U> = Object with M<U>;
+class A<U> = Object with M<U>;
 
-typedef B<V> = Object with A<V>;
+class B<V> = Object with A<V>;
 
-typedef C<U> = Object with M<List<U>>;
+class C<U> = Object with M<List<U>>;
 
-typedef D<V> = Object with C<Set<V>>;
+class D<V> = Object with C<Set<V>>;
 
 class E extends A<num> { }
 
 class F extends B<String> { }
 
 class G<T> extends C<T> { }
 
 class H<T> extends D<Map<String, T>> { }
 
 main() {
   Expect.equals("num", new E().t().toString());
   Expect.equals("String", new F().t().toString());
   Expect.equals("List<bool>", new G<bool>().t().toString());
   Expect.equals("List<Set<Map<String, int>>>", new H<int>().t().toString());
 }