Revert various changes to Link factories.

dart/frog/leg/util/link_implementation.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.
 
-class LinkFactory  {
-  static Link createLink(head, [Link tail]) {
+// TODO(ahe): This class should not be generic.
+class LinkFactory<T> {
+  factory Link(head, [Link tail]) {
     return new LinkEntry(head, (tail === null) ? const LinkTail() : tail);
   }
 
-  static Link createFromList(List list) {
+  factory Link.fromList(List list) {
     switch (list.length) {
       case 0:
         return const LinkTail();
       case 1:
-        return createLink(list[0]);
+        return new Link(list[0]);
       case 2:
-        return createLink(list[0], createLink(list[1]));
+        return new Link(list[0], new Link(list[1]));
       case 3:
-        return createLink(list[0], createLink(list[1], createLink(list[2])));
+        return new Link(list[0], new Link(list[1], new Link(list[2])));
     }
-    Link link = createLink(list.last());
+    Link link = new Link(list.last());
     for (int i = list.length - 1; i > 0; i--) {
       link = link.prepend(list[i - 1]);
     }
     return link;
   }
 }
 
 class AbstractLink<T> implements Link<T> {
   T get head() { throw "bug"; } // TODO(ahe): Work around VM bug.
   T get tail() { throw "bug"; } // TODO(ahe): Work around VM bug.
   abstract List<T> toList(); // TODO(ahe): Work around Frog bug #318.
   abstract bool isEmpty(); // TODO(ahe): Work around Frog bug #318.
 
   const AbstractLink();
 
   Link<T> prepend(T element) {
-    return LinkFactory.createLink(element, this);
+    return new Link<T>(element, this);
   }
 
   Iterator<T> iterator() => toList().iterator();
 
   void printOn(StringBuffer buffer, [separatedBy]) {
     if (isEmpty()) return;
     // TODO(ngeofray): Work around Frog bug
     buffer.add(head === null ? 'null' : head);
     if (separatedBy === null) separatedBy = '';
     for (Link link = tail; !link.isEmpty(); link = link.tail) {
@@ skipping 66 matching lines @@
   void addLast(T t) {
     LinkEntry<T> entry = new LinkEntry<T>(t, null);
     if (head === null) {
       head = entry;
     } else {
       lastLink.realTail = entry;
     }
     lastLink = entry;
   }
 }