Port Sky widgets demo to Dart

Port Sky widgets demo to Dart

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

Committed: https://chromium.googlesource.com/external/mojo/+/9350c53370050ad02fb93f84ba0ea476e83a7fdc

[eseidel, 2015-02-23 16:01:48]

eseidel@chromium.org changed reviewers:
+ eseidel@chromium.org

[eseidel, 2015-02-23 16:01:48]

lgtm

[abarth-chromium, 2015-02-23 16:30:41]

Committed patchset #1 (id:1) manually as
9350c53370050ad02fb93f84ba0ea476e83a7fdc (presubmit successful).