code: @Nonnull
    public static FieldValue arrayUnion(@Nonnull Object... elements) {
      Preconditions.checkArgument(elements.length > 0, "arrayUnion() expects at least 1 element");
      return new ArrayUnionFieldValue(Arrays.asList(elements));
    }
docs: Returns a special value that can be used with set(), create() or update() that tells the server
  to union the given elements with any array value that already exists on the server. Each
  specified element that doesn't already exist in the array will be added to the end. If the
  field being modified is not already an array it will be overwritten with an array containing
  exactly the specified elements.
  
  @param elements The elements to union into the array.
  @return The FieldValue sentinel for use in a call to set() or update().