Creates a new zone as a child of this.
The new zone will have behavior like the current zone, except where
overridden by functions in specification.
The new zone will have the same stored values (accessed through
operator []) as this zone, but updated with the keys and values
in zoneValues. If a key is in both this zone's values and in
zoneValues, the new zone will use the value from `zoneValues``.
Source
Zone fork({ ZoneSpecification specification,
Map zoneValues });