Allow the list of type parameters in the default factory clause to be omitted.

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> default Factory<T> {
+// The list of type parameters in the default factory clause can be omitted.
+
+interface Interface<T> default Factory {
   Interface();
   Interface.withArg(T value);
 }
 
 class Factory<T> {
   factory Interface() {
     return null;
   }
 
   factory Interface.withArg(value) {
     return null;
   }
 }
 
 main() {
   new Interface();
   new Interface.withArg(4);
 }