Remove all warnings from dart2js and add a test to prevent introduction of new.

dart/sdk/lib/_internal/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.
 
 part of js;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 131 matching lines @@
   T visitProperty(Property node) => visitNode(node);
   T visitRegExpLiteral(RegExpLiteral node) => visitExpression(node);
 }
 
 abstract class Node {
   var sourcePosition;
   var endSourcePosition;
 
   accept(NodeVisitor visitor);
   void visitChildren(NodeVisitor visitor);
+
+  VariableUse asVariableUse() => null;
 }
 
 class Program extends Node {
   final List<Statement> body;
   Program(this.body);
 
   accept(NodeVisitor visitor) => visitor.visitProgram(this);
   void visitChildren(NodeVisitor visitor) {
     for (Statement statement in body) statement.accept(visitor);
   }
@@ skipping 488 matching lines @@
 
   accept(NodeVisitor visitor);
   int get precedenceLevel => PRIMARY;
   void visitChildren(NodeVisitor visitor) {}
 }
 
 class VariableUse extends VariableReference {
   VariableUse(String name) : super(name);
 
   accept(NodeVisitor visitor) => visitor.visitVariableUse(this);
+
+  VariableUse asVariableUse() => this;
 }
 
 class VariableDeclaration extends VariableReference {
   VariableDeclaration(String name) : super(name);
 
   accept(NodeVisitor visitor) => visitor.visitVariableDeclaration(this);
 }
 
 class Parameter extends VariableDeclaration {
   Parameter(String id) : super(id);
@@ skipping 176 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;
 }