Properly exposing NodeIterator and TreeWalker.

Properly exposing NodeIterator and TreeWalker

Also removing references to EntityReference which is an outdated XML type.

Note that there's a bit of an oddity here around NodeFilter, which is both a class with all of the filter flags, and a function signature for a callback.

For now, I've scrubbed references to NodeFilter (the callback), but if desired I can add a new filter callback. This will most likely require a bit of Dartium work as well, so a separate CL I think.

BUG=
R=jmesserly@google.com

Committed: https://code.google.com/p/dart/source/detail?r=22160

[Jennifer Messerly, 2013-04-09 02:22:40]

lgtm. Funny that the types already existed, they just weren't constructable.

[blois, 2013-04-30 01:36:11]

Committed patchset #2 manually as r22160 (presubmit successful).