js: fix core.Object, output order, static const fields

lib/src/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_ast;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 1167 matching lines @@
   final VariableDeclaration name;
   final Expression heritage; // Can be null.
   final List<Method> methods;
 
   ClassExpression(this.name, this.heritage, this.methods);
 
   accept(NodeVisitor visitor) => visitor.visitClassExpression(this);
 
   void visitChildren(NodeVisitor visitor) {
     name.accept(visitor);
-    heritage.accept(visitor);
+    if (heritage != null) heritage.accept(visitor);
     for (Method element in methods) element.accept(visitor);
   }
 
   ClassExpression _clone() => new ClassExpression(name, heritage, methods);
 
   int get precedenceLevel => PRIMARY;
 }
 
 class Method extends Property {
   final bool isGetter;
@@ skipping 207 matching lines @@
   final Expression expression;
 
   CommentExpression(this.comment, this.expression);
 
   int get precedenceLevel => PRIMARY;
   accept(NodeVisitor visitor) => visitor.visitCommentExpression(this);
   CommentExpression _clone() => new CommentExpression(comment, expression);
 
   void visitChildren(NodeVisitor visitor) => expression.accept(visitor);
 }