The build strategy currently used by this builder.
ConnectionState.waiting: future is not null, but has not yet completed. The AsyncSnapshot.data will be set to initialData, unless a future has previously completed, in which case the previous result persists.
ConnectionState.done: future is not null, and has completed. If the future completed successfully, the AsyncSnapshot.data will be set to the value to which the future completed. If it completed with an error, AsyncSnapshot.hasError will be true and AsyncSnapshot.error will be set to the error object.
This builder must only return a widget and should not have any side effects as it may be called multiple times.
final AsyncWidgetBuilder<T> builder;