Make LinkBuilder an abstract class.

sdk/lib/_internal/compiler/implementation/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 org_dartlang_compiler_util;
 
 class Link<T> implements Iterable<T> {
   T get head => null;
   Link<T> get tail => null;
 
@@ skipping 41 matching lines @@
   void forEach(void f(T element)) {}
 
   bool operator ==(other) {
     if (other is !Link<T>) return false;
     return other.isEmpty;
   }
 
   String toString() => "[]";
 }
 
-interface LinkBuilder<T> default LinkBuilderImplementation<T> {
-  LinkBuilder();
+abstract class LinkBuilder<T> {
+  factory LinkBuilder() = LinkBuilderImplementation;
 
   Link<T> toLink();
   void addLast(T t);
 
   final int length;
 }