restore dirName. It was added in html5

restore dirName. It was added in html5:

http://developers.whatwg.org/association-of-controls-and-forms.html#attr-fe-dirname
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-input-element.html
https://bugs.webkit.org/show_bug.cgi?id=76766

Committed: https://code.google.com/p/dart/source/detail?r=14239

[Jennifer Messerly, 2012-10-26 23:02:27]

I think this was added to the "removed DOM fields" by accident. We probably
don't want to be removing new stuff that html5 compliance tests are looking for.

[blois, 2012-10-26 23:55:35]

lgtm

[Jacob, 2012-10-27 01:15:01]

lgtm