[refactored] aggregators to use combineLatest rather than emptyStream
[refactored] aggregators to use combineLatest rather than emptyStream

Summary: `combineLatest` exists now, so we don't need to hack `$._reactiveMap()` to simulate it.

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/D3391

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