where method
only match param when its text matches regex, see Route.where.
A leading ^ and a trailing $ are dropped: the regex must match
the whole param text anyway, and inside the route regex they would
never match.
Implementation
void where(String param, String regex) {
(wheres ??= <String, String>{})[param] = _withoutAnchors(regex);
_compiledFor = null;
_whereChecks = null;
}