[_changed] internal property to have leading underscore
[_changed] internal property to have leading underscore

Reviewers: #mdm_core_contributors, markwei

Reviewed By: #mdm_core_contributors, markwei

Subscribers: markwei

Differential Revision: http://codereview.cc/D1202

material-motion/material-motion-js,material-motion/material-motion-js,material-motion/material-motion-js