getFirstWhitespaceIndexAfterLineNumber(lineNumber) {
        this._checkPendingChanges();
        lineNumber = lineNumber | 0;
        return this._findFirstWhitespaceAfterLineNumber(lineNumber);
    }
Find the index of the first whitespace which has `afterLineNumber` >= `lineNumber`.
@return The index of the first whitespace with `afterLineNumber` >= `lineNumber` or -1 if no whitespace is found.