Fixes bad code gen on non-widget children in DockLayoutPanelParser, and allows
<g:center> to appear anywhere.

To allow real unit testing, refactored UiBinderWriter to be a bit more DI. This
included moving w3c parsing knowledge outside of the writer. In the process made
the w3c dom test helper into the actual production code--it was nearly identical
anyway, and now test and prod won't drift.

This took a little while. But the next one won't.

Review by jgw, scottb
http://gwt-code-reviews.appspot.com/93806

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@6714 8db76d5a-ed1c-0410-87a9-c151d255dfc7
16 files changed