function getValueForKey(key) {
      if (next.hasOwnProperty(key)) {
        return next[key];
      } else {
        return prev[key];
      }
    }
When you're adding or removing children some may be added or removed in the
same render pass. We want to show *both* since we want to simultaneously
animate elements in and out. This function takes a previous set of keys
and a new set of keys and merges them with its best guess of the correct
ordering. In the future we may expose some of the utilities in
ReactMultiChild to make this easy, but for now React itself does not
directly have this concept of the union of prevChildren and nextChildren
so we implement it here.

@param {object} prev prev children as returned from
`ReactTransitionChildMapping.getChildMapping()`.
@param {object} next next children as returned from
`ReactTransitionChildMapping.getChildMapping()`.
@return {object} a key set that contains all keys in `prev` and all keys
in `next` in a reasonable order.