destroyInstance method
Remove the instance from internal isolate dictionary and make it available for garbage collection
Implementation
void destroyInstance(PooledInstanceProxy instance) {
var index = indexOfPi(instance);
if (index == -1) {
throw 'Cant find instance with id ${instance._instanceId} among active to destroy it';
}
_isolateSendPorts[index]!.send(_DestroyRequest(instance._instanceId));
_pooledInstances.remove(instance._instanceId);
}