Support runtime check of function types.

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> extends Iterable<T> {
   T get head => null;
   Link<T> get tail => null;
 
@@ skipping 55 matching lines @@
   get length {
     throw new UnsupportedError('get:length');
   }
 
   int slowLength() => 0;
 }
 
 abstract class LinkBuilder<T> {
   factory LinkBuilder() = LinkBuilderImplementation;
 
-  Link<T> toLink();
+  /**
+   * 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()]);
+
   void addLast(T t);
 
   final int length;
   final bool isEmpty;
 }