Called to obtain a precise measure of the total number of children.
Must return the number that is one greater than the greatest
createChild will actually create a child.
This is used when createChild cannot add a child for a positive
to determine the precise dimensions of the sliver. It must return an
accurate and precise non-null value. It will not be called if
createChild is always able to create a child (e.g. for an infinite
int get childCount;