Creates a new zone as a child of this.
The new zone uses the closures in the given specification to override
the current's zone behavior. All specification entries that are null
inherit the behavior from the parent zone (this).
The new zone inherits the stored values (accessed through [])
of this zone and updates them with values from zoneValues, which either
adds new values or overrides existing ones.
Note that the fork operation is interceptible. A zone can thus change the zone specification (or zone values), giving the forking zone full control over the child zone.
Source
Zone fork({ZoneSpecification specification, Map zoneValues});