function testUnique() {
  index = new lf.index.NullableIndex(
      new lf.index.BTree(
      'test',
      new lf.index.SimpleComparator(lf.Order.ASC),
      /* opt_unique */ true));
  index.add(null, 1);
  index.add(1, 2);
  index.add(null, 3);

  assertArrayEquals([1, 3], index.get(null));
}
Tests that a unique nullable index allows multiple nullable keys (this
matches the behavior of other SQL engines).