function getLineNumber(src, pattern) {
  const index = stringSearch(src, pattern);
  if (index < 0) {
    return -1;
  }

  // The importPattern incidentally captures an initial \n in
  // an attempt to reject a . prefix, so we need to offset
  // the line number in that case.
  const adjustment = src[index] === '\n' ? 1 : 0;

  return stringSplit(stringSlice(src, 0, index), '\n').length + adjustment;
}
Find the first occurence of the given pattern and return
the location as the approximate line number.

@param {string} src
@param {RegExp} pattern
@returns {number}