fixes #168, dart:js implementation with a test

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 908 matching lines @@
 class RestParameter extends Expression implements Parameter {
   final Identifier parameter;
 
   RestParameter(this.parameter);
 
   RestParameter _clone() => new RestParameter(parameter);
   accept(NodeVisitor visitor) => visitor.visitRestParameter(this);
   void visitChildren(NodeVisitor visitor) {
     parameter.accept(visitor);
   }
-  int get precedenceLevel => SPREAD;
+  int get precedenceLevel => PRIMARY;
 }
 
 class This extends Expression {
   accept(NodeVisitor visitor) => visitor.visitThis(this);
   This _clone() => new This();
   int get precedenceLevel => PRIMARY;
   void visitChildren(NodeVisitor visitor) {}
 }
 
 // `super` is more restricted in the ES6 spec, but for simplicity we accept
@@ skipping 522 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);
 }