code: function(one, two) {
      invariant(
        Array.isArray(one) && Array.isArray(two),
        'Tried to merge arrays, instead got %s and %s.',
        one,
        two
      );
    }
docs: If merging Arrays, a merge strategy *must* be supplied. If not, it is
  likely the caller's fault. If this function is ever called with anything
  but `one` and `two` being `Array`s, it is the fault of the merge utilities.
  
  @param {*} one Array to merge into.
  @param {*} two Array to merge from.