This patch starts down the process of cleaning up the dart:ui API. In this
approach, we use Dart code to specify the interface instead of IDL. The code in
this patch is far from complete. It's just a sketch to see how this sort of
approach might work.
Remove all code relating to shadow trees, insertion points, shadow
boundaries, traversing composed trees, distribution, template
documents, custom elements, registering elements, element registries,
element factories, shadow roots, etc.
Remove the following features from the IDLs and from the binding
generators: CustomElementCallbacks, Reflect*, EventHandler.
Remove the CSS custom pseudo-element concept, since we no longer have
a UA style sheet worth talking about, no longer have shadow trees or
custom elements, no longer use pseudo-elements, and generally
therefore don't use this code at all.
