use const Link so that empty Link is always identical

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 43 matching lines @@
   Iterator<T> get iterator {
     return new MappedLinkIterator<S, T>(_link, _transformation);
   }
 }
 
 class LinkEntry<T> extends Link<T> {
   final T head;
   Link<T> tail;
 
   LinkEntry(T this.head, [Link<T> tail])
-      : this.tail = ((tail == null) ? new Link<T>() : tail);
+      : this.tail = ((tail == null) ? const Link() : tail);
 
   Link<T> prepend(T element) {
     // TODO(ahe): Use new Link<T>, but this cost 8% performance on VM.
     return new LinkEntry<T>(element, this);
   }
 
   void printOn(StringBuffer buffer, [separatedBy]) {
     buffer.write(head);
     if (separatedBy == null) separatedBy = '';
     for (Link link = tail; link.isNotEmpty; link = link.tail) {
@@ skipping 134 matching lines @@
     }
     throw new StateError("no elements");
   }
 
   void clear() {
     head = null;
     lastLink = null;
     length = 0;
   }
 }