protected static TestingUnbufferedCharStream createStream(String text, int bufferSize) {
		return new TestingUnbufferedCharStream(new StringReader(text), bufferSize);
	}
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