Add destinations operator for Transition.Direction.
Add destinations operator for Transition.Direction.

Summary:
Used to build reversible transitions.

Depends on D2414

Reviewers: O2 Material Motion, #material_motion, O4 Material Apple platform reviewers, chuga

Reviewed By: O4 Material Apple platform reviewers, chuga

Tags: #material_motion

Differential Revision: http://codereview.cc/D2415

material-motion/material-motion-swift,material-motion/material-motion-swift