title: flutter - SliverList / SliverChildBuilderDelegate supply initial index or allow negative indices
body: <p>I'm currently building a calendar view in Flutter using a SliverList and a SliverChildBuilderDelegate such that I don't have to render every single item in the calendar at once.</p>
  <p>The first date is epoch time, Jan 1, 1970, and the last date is some odd amount of time computed after today's date.</p>
  <p>My issue is is that when I first render the view, I want it to render the view starting today, not on Jan 1, 1970. However, if I have today as the 0 indexes, negative indices are not allowed (or supplied) to the builder delegate so you can't scroll up from that date. You also can't supply an initial index, as far as I can tell, to the builder or the list, so I can't make epoch time as the 0 indexes either since the list will just start there, making for quite the terrible experience! I'm not entirely sure how to proceed.</p>
  <p>Does anyone have any suggestions?</p>