Support serialization of the compiler backbone.

pkg/compiler/lib/src/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.
 
 part of util_implementation;
 
 class LinkIterator<T> implements Iterator<T> {
   T _current;
   Link<T> _link;
 
@@ skipping 162 matching lines @@
     while (!link.isEmpty) {
       list[index] = link.head;
       link = link.tail;
       index++;
     }
     lastLink = null;
     head = null;
     return list;
   }
 
-  void addLast(T t) {
+  Link<T> addLast(T t) {
     length++;
     LinkEntry<T> entry = new LinkEntry<T>(t, null);
     if (head == null) {
       head = entry;
     } else {
       lastLink.tail = entry;
     }
     lastLink = entry;
+    return entry;
   }
 
   bool get isEmpty => length == 0;
 }