[added] validation that a value is an Event in isPointerEvent
[added] validation that a value is an Event in isPointerEvent

Summary: To ensure we don't call `event.target.setPointerCapture(event.pointerId)` on a fake `PointerEvent`.

Reviewers: O2 Material Motion, O3 Material JavaScript platform reviewers, #material_motion, featherless

Reviewed By: O2 Material Motion, #material_motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D3086

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