All reflected attributes two way bind on SkyElement, so now doing
<sky-element name="sky-input" attributes="value:string"> is enough
to get two way binding on the value attribute so users doing
<sky-input value="{{ inputValue }}"> will get the inputValue property
updated as the user types.

R=abarth@chromium.org, ojan@chromium.org

Review URL: https://codereview.chromium.org/850383002