Presence of comment tags in the Source breaks range selection

[dgaspar]

How to reproduce:

1. open the demo,
2. go to Source mode and paste this:

   <p><!-- BUG -->loren ipsum</p>
   

3. go back to WYSIWYG,
4. press ctrl+A (select all), ctrl+B (for bold) --> BUG!

A quick fix: core/dom/range.js, enlarge() method:

if ( ( sibling.$.offsetWidth > 0 || excludeBrs && sibling.is && sibling.is( 'br' ) ) && !sibling.data( 'cke-bookmark' ) )

The comment nodes are represented by CKEDITOR.dom.node (which doesn't have the is() method) instead of the expected CKEDITOR.dom.element

[j.swiderski]

DUP of #8732