Fix factories in Frog to correspond to the new syntax.

frog/leg/util/link.dart

Index: frog/leg/util/link.dart
diff --git a/frog/leg/util/link.dart b/frog/leg/util/link.dart
index d43a6b8a563fc65d527865f9a65b8342bef6d4de..4d0c8852e9195b98fc3b13864f3a6c096f43856a 100644
--- a/frog/leg/util/link.dart
+++ b/frog/leg/util/link.dart
@@ -2,7 +2,7 @@
 // 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 Link<T> extends Iterable<T> factory LinkFactory {
+interface Link<T> extends Iterable<T> default LinkFactory<T> {
   final T head;
   final Link<T> tail;
 
@@ -17,11 +17,11 @@ interface Link<T> extends Iterable<T> factory LinkFactory {
   void printOn(StringBuffer buffer, [separatedBy]);
 }
 
-interface EmptyLink<T> extends Link<T> factory LinkTail<T> {
+interface EmptyLink<T> extends Link<T> default LinkTail<T> {
   const EmptyLink();
 }
 
-interface LinkBuilder<T> factory LinkBuilderImplementation<T> {
+interface LinkBuilder<T> default LinkBuilderImplementation<T> {
   LinkBuilder();
 
   Link<T> toLink();