These tweaks came from https://github.com/flutter/packages/pull/4536

- The TwoDimensionalChildBuilderDelegate asserts that maxXIndex and maxYIndex are null or >= 0
- The TwoDimensionalChildDelegate setter in RenderTwoDimensionalViewport has a covariant to allow type safety for subclasses of RenderTwoDimensionalViewport implementing with other subclasses of TwoDimensionalChildDelegate

I'd like to cherry pick this so https://github.com/flutter/packages/pull/4536 will not have to wait for it to reach stable.