This CL updates sky-box, sky-button, sky-checkbox, sky-input, and sky-radio to
work in Dart. We don't have a data binding system yet, so there's a bit more
plumbing in the code.

This CL adds support for sky-element@attributes, which lets you specify which
attributes your element supports. We use this information to synthesize getters
and setters for those attributes and to dispatch to mumbleChanged methods when
the attributes change.

I've also wrapped the widgets demo itself in a sky-scrollable so the whole
thing scrolls.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/946813005