_ensureTarget, when inside a custom PaperDialogBehavior element does not work

[wesalvaro]

Description

Resizing when the part of a custom element does not work.
_ensureTarget does not work when added as the first child of a custom element.
Workaround is to set the custom element's sizingTarget in the ready function to the paper-dialog-scrollable. It looks like the dialog element is getting set to the document fragment and not the element. Perhaps setting

Expected outcome

When resizing, the contents of the dialog should not escape the dialog.
this.dialogElement should be set to the custom element.

Actual outcome

When resizing, the contents of the dialog escape the dialog.
this.dialogElement is set to the shadow-root/document fragment of the custom element.

Live Demo

https://output.jsbin.com/pimamozebi

Steps to reproduce

Create custom element with the Polymer.PaperDialogBehavior and put a paper-dialog-scrollable as its only child in the template. Notice that resizing does not work as it should.
Using this.parentNode.host, if it exists, fixes the issue.

Browsers Affected

• Chrome