Add tests for Element#elements.

Add tests for Element#elements.
    
This also factors out some of the range functions in NodeList and adds them to
_Lists. _Lists was then renamed to Lists so it could be accessed from the
testing code.
    
This was originally submitted as r3320. That was rolled back due to errors in
the dartium bot, but the logs are unavailable and local tests seem to pass so
we're rolling forward again.

TBR

Committed: https://code.google.com/p/dart/source/detail?r=3374

[Jacob, 2012-01-18 19:23:51]

lgtm