Support runtime check of function types.

sdk/lib/_internal/compiler/implementation/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 92 matching lines @@
   int slowLength() => 1 + tail.slowLength();
 }
 
 class LinkBuilderImplementation<T> implements LinkBuilder<T> {
   LinkEntry<T> head = null;
   LinkEntry<T> lastLink = null;
   int length = 0;
 
   LinkBuilderImplementation();
 
-  Link<T> toLink() {
-    if (head == null) return const Link();
-    lastLink.tail = const Link();
+  Link<T> toLink([Link<T> tail = const Link()]) {
+    if (head == null) return tail;
+    lastLink.tail = tail;
     Link<T> link = head;
     lastLink = null;
     head = null;
     return link;
   }
 
   void addLast(T t) {
     length++;
     LinkEntry<T> entry = new LinkEntry<T>(t, null);
     if (head == null) {
       head = entry;
     } else {
       lastLink.tail = entry;
     }
     lastLink = entry;
   }
 
   bool get isEmpty => length == 0;
 }