code: @CanIgnoreReturnValue
    private boolean finishLine(boolean sawNewline) throws IOException {
      String separator = sawReturn ? (sawNewline ? "\r\n" : "\r") : (sawNewline ? "\n" : "");
      handleLine(line.toString(), separator);
      line = new StringBuilder();
      sawReturn = false;
      return sawNewline;
    }
docs: Called when a line is complete.