It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

Fixes #11855

Also, fix some logic in SliverPadding.

This patch passes down the viewportMainAxisExtent so that RenderSliverPage can
pick it up and size its children appropriately.

This widget is a replacement for ScrollableViewport that uses the new
Scrollable2 machinery. The widget is not based on Slivers but does use the new
scroll behavior classes.