Added TODO.

lib/compiler/implementation/js/nodes.dart

 // Copyright (c) 2012, 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.
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
   T visitExpressionStatement(ExpressionStatement node);
   T visitEmptyStatement(EmptyStatement node);
@@ skipping 123 matching lines @@
   T visitLiteralNumber(LiteralNumber node) => visitLiteral(node);
   T visitLiteralNull(LiteralNull node) => visitLiteral(node);
 
   T visitArrayInitializer(ArrayInitializer node) => visitExpression(node);
   T visitArrayElement(ArrayElement node) => visitNode(node);
   T visitObjectInitializer(ObjectInitializer node) => visitExpression(node);
   T visitProperty(Property node) => visitNode(node);
   T visitRegExpLiteral(RegExpLiteral node) => visitExpression(node);
 }
 
-class Node {
+abstract class Node {
   var sourcePosition;
   var endSourcePosition;
 
   abstract accept(NodeVisitor visitor);
   abstract void visitChildren(NodeVisitor visitor);
 }
 
 class Program extends Node {
   final List<Statement> body;
   Program(this.body);
@@ skipping 675 matching lines @@
   /** Contains the pattern and the flags.*/
   String pattern;
 
   RegExpLiteral(this.pattern);
 
   accept(NodeVisitor visitor) => visitor.visitRegExpLiteral(this);
   void visitChildren(NodeVisitor visitor) {}
 
   int get precedenceLevel => PRIMARY;
 }