Implement <sky-input>
Implement <sky-input>

This CL contains a basic input element.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/831353003

cbracken/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,aghassemi/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,jason-simmons/flutter,jason-simmons/flutter,flutter/flutter,aghassemi/flutter,tvolkert/flutter,aghassemi/flutter,cbracken/flutter,turnidge/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,jason-simmons/flutter,turnidge/flutter,turnidge/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,turnidge/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,aghassemi/flutter,jason-simmons/flutter,cbracken/flutter,turnidge/flutter