Window.open method should return a Window, not a WindowBase, so the opened window's document.body can be manipulated

[DartBot]

This issue was originally filed by silviu...@gmail.com

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What steps will reproduce the problem?

1. Open Dart Editor;
2. Create a web application project;
3. Write this in the .dart file: window.open('', 'test').document.body.

What is the expected output? What do you see instead?
The editor warns me that window.open returns a WindowBase which does not have a 'document' property.

What version of the product are you using?
Dart SDK 1.5.8

On what operating system?
Ubuntu 14.04 x64

What browser (if applicable)?
N/A

Please provide any additional information below.
It is useful to be able to manipulate the body of the opened (created) window and I do not see a reason for which this browser API is not exposed.

[DartBot]

This comment was originally written by @zoechi

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see also
https://code.google.com/p/dart/issues/detail?id=20216

[DartBot]

This comment was originally written by silviu...@gmail.com

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I have found a hacky way to do what I want but I must do two unintuitive forced casts:

    Window w = window.open('', 'test') as Window;
    (w.document as HtmlDocument).body.appendHtml('<h1>Test</h1>');

I am convinced that this part of the API can be improved.

[DartBot]

This comment was originally written by @zoechi

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This is not hacky at all but doesn't work anymore because after a recent fix
'_DomWindowCrossFrame' is returned instead of a 'WindowBase' which doesn't allow to access to 'document' or any other method/field of 'Window'.

[sgjesse]

Added Area-Library, Library-Html, Triaged labels.

[vsmenon]

Added Duplicate label.
Marked as being merged into #20146.