Returns the URI query split into a map according to the rules specified for FORM post in the HTML 4.01 specification section 17.13.4. Each key and value in the returned map has been decoded. If there is no query the empty map is returned.
Keys in the query string that have no value are mapped to the empty string.
The returned map is unmodifiable and will throw UnsupportedError on any calls that would mutate it.