Revert various changes to Link factories.

dart/tests/language/src/NonParameterizedFactoryTest.dart

 // Copyright (c) 2011, 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.
 
 interface Interface<T> factory Factory {
   Interface();
   Interface.withArg(T value);
 }
 
-// A factory must be parametrized. Use legacy form.
 class Factory {
   factory Interface<T>() {
     return null;
   }
 
   factory Interface<T>.withArg(value) {
     return null;
   }
 }
 
 main() {
   new Interface();
   new Interface.withArg(4);
 }