add method
Adds a protected resource.
uri- a regular expression used to match the request URI.
Note: add appends; the first matching mapping wins in canAccess.
To replace an existing entry, call remove first.
Implementation
void add(String uri, Iterable<String> roles) {
assert(_mapping.every((m) => m.pattern.pattern != "^$uri\$"),
"Duplicate URI pattern: $uri (call remove() first to replace)");
_mapping.add(_ACMapping(uri, roles));
}