@Test
	public void testDanglingHighSurrogateThrows() {
		assertThrows(RuntimeException.class, () -> createStream("\uD83C\u0123"));
	}
It is not valid to pass a mark to {@link IntStream#seek}, but
{@link UnbufferedCharStream} creates marks in such a way that this
invalid usage results in an {@link IllegalArgumentException}.
tool-testsuite/test/org/antlr/v4/test/tool/TestUnbufferedCharStream.java