The number of children that will contribute semantic information.
The value will be null if the number of children is unknown or unbounded.
Some subtypes of ScrollView can infer this value automatically. For example ListView will use the number of widgets in the child list, while the new ListView.separated constructor will use half that amount.
For CustomScrollView and other types which do not receive a builder or list of widgets, the child count must be explicitly provided.
final int semanticChildCount