* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Fix SliverPadding dropping the chain on scrollOffsetCorrection propagation

* review doc

* Fix the confusing-zero case with NestedScrollView.

* Update mock_canvas.dart

* Update tabs_demo.dart

* more tweaks

This patch includes:

 * SliverPadding
 * ScrollView
 * PageView

Fixes #11764
Fixes #11836

Fix sliver padding assertion failure that sometimes happens during startup because the app is temporarily built with zero size

Also, fix some logic in SliverPadding.

* add @required when there's an assert not null

* address review comments

Also, add a few other random dartdocs that I encountered along the way.

Previously the "left" padding was applied on the right when hit testing
a reversed vertical list view.

...for the case where there's no right or bottom padding.

This patch makes Scrollable2.ensureVisible with SingleChildScrollView. A future
patch will extend the implementation to work with slivers. (Although the patch
does include some of the infrastructure for that part of the implementation as
well.)

This patch adds grid supports to slivers and introduces a ScrollGrid
convenience class for making the common types of scrollable grids.

This patch also deploys ScrollGrid in an example in the Flutter Gallery.

When you put a box in a Viewport2, it might paint outside its allocated bounds.
This patch teaches Viewport2 to clip when that happens.