This issue was originally filed by igor.ko...@mailvision.com

What steps will reproduce the problem?

1. open dart editor
2. new application/web application (with generate sample content)
3. add to main()
     window.onBeforeUnload.listen((e)=>"Good Bye");
4. run it in Dartium mode and close tab, then run it as JavaScript and
   close tab

What is the expected output? What do you see instead?

in Dartium it works as should: pop up window with
"Confirm Navigation. Good Bye. Are you sure you want to leave this page" and buttons "Leave this Page" and "Stay on this Page"

in run as JavaScript page closed without the pop up window.

It's bug in window.onBeforeUnload implementation, because
if you replace it to:

import 'dart:js';

context["onbeforeunload"] = ((e)=>"Good Bye");

It works both in Dartium and run as JavaScript.

What version of the product are you using? On what operating system?
Dart SDK version 1.1.0-dev.5.11 Windows 7 64 bits

Please provide any additional information below.