// Two differing invalid pairs.
    assertEquals("abc\uD8AB", Strings.commonPrefix("abc\uD8AB\uD8ABdef", "abc\uD8AB\uD8ACxyz"));
    // One orphan high surrogate.
    assertEquals("", Strings.commonPrefix("\uD8AB\uDCAB", "\uD8AB"));
    // Two orphan high surrogates.
    assertEquals("\uD8AB", Strings.commonPrefix("\uD8AB", "\uD8AB"));
  }