I had to add back document.createText() since new Text()
does not work in the new world yet.

LayoutRoot is a new Dart-exposed class which holds the Frame and
all associated machinery, sufficient to trigger a restyle
and layout of the subtree.

This is all kinda hacky and I'm sure likely to cause many
crashes if folks call random methods on these disconnected
trees.

But this makes it at least possible to paint text for now
and we can improve this in the coming days.

This really should have Adam's review.  It's hugely hacky
but I'd like to negotiate out with him the timeline on
which we should fix some of these hacks.

R=ianh@google.com
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/1148253003