@Test public void test1Char() throws Exception {
		TestingUnbufferedCharStream input = createStream("x");
		assertEquals('x', input.LA(1));
		input.consume();
		assertEquals(IntStream.EOF, input.LA(1));
		String r = input.getRemainingBuffer();
		assertEquals("\uFFFF", r); // shouldn't include x
		assertEquals("\uFFFF", input.getBuffer()); // whole buffer
	}
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