Support serialization of the compiler backbone.

pkg/compiler/lib/src/util/link.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 dart2js.util;
 
 class Link<T> {
   T get head => throw new StateError("no elements");
   Link<T> get tail => null;
 
@@ skipping 114 matching lines @@
   factory LinkBuilder() = LinkBuilderImplementation;
 
   /**
    * Prepends all elements added to the builder to [tail]. The resulting list is
    * returned and the builder is cleared.
    */
   Link<T> toLink([Link<T> tail = const Link()]);
 
   List<T> toList();
 
-  void addLast(T t);
+  Link<T> addLast(T t);
 
   final int length;
   final bool isEmpty;
 }