path is a root-relative path and
false if it's not.
URLs that start with
/ are known as "root-relative", since they're
relative to the root of the current URL. Since root-relative paths are still
absolute in every other sense, isAbsolute will return true for them. They
can be detected using isRootRelative.
No POSIX and Windows paths are root-relative.
bool isRootRelative(String path) => context.isRootRelative(path);