Revert various changes to Link factories.

dart/frog/leg/tree/nodes.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.
 
 interface Visitor<R> {
   R visitBlock(Block node);
   R visitClassNode(ClassNode node);
   R visitExpressionStatement(ExpressionStatement node);
   R visitFor(For node);
   R visitFunctionExpression(FunctionExpression node);
@@ skipping 146 matching lines @@
 }
 
 class NodeList extends Node {
   final Link<Node> nodes;
   final Token beginToken;
   final Token endToken;
   final SourceString delimiter;
 
   const NodeList([this.beginToken, this.nodes, this.endToken, this.delimiter]);
 
-  NodeList.singleton(Node node) : this(null, LinkFactory.createLink(node));
+  NodeList.singleton(Node node) : this(null, new Link<Node>(node));
 
   accept(Visitor visitor) => visitor.visitNodeList(this);
 
   Token getBeginToken() {
     if (beginToken !== null) return beginToken;
      if (nodes !== null) {
        for (Link<Node> link = nodes; !link.isEmpty(); link = link.tail) {
          if (link.head.getBeginToken() !== null) {
            return link.head.getBeginToken();
          }
@@ skipping 259 matching lines @@
 }
 
 /** Representation of modifiers such as static, abstract, final, etc. */
 class Modifiers {
   final Link<Token> modifiers;
   /** Bit pattern to easy check what modifiers are present. */
   final int flags;
 
   const Modifiers([this.modifiers, this.flags = 0]);
 }