Fixes#158158
Similar to 154484, but addressing more fundamental causes. We should call the child's `didUpdateWidget` before the child's `build` when states change.
1. Remove redundant code but keep the old test because we don't need it after this fix.
2. Add getters of `lastShrinkOffset` and `lastOverlapsContent` that are needed when `SliverAppBar.update`.
3. Add a test with a custom TabBar to avoid breaking when some changes to the default TabBar.
5f0f18d739