Creates a resource object with the given
uri as location.
uri is a string containing a valid URI.
If the string is not a valid URI, using any of the functions on
the resource object will fail.
The URI may be relative, in which case it will be resolved against Uri.base before being used.
The URI may use the
package scheme, which is always supported.
Other schemes may also be supported where possible.
external const factory Resource(String uri);