* some space formattings

* always use blocks in if-else if a block is used

* format spaces in for and while

* allow multiline if conditions

* fix missing space

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

Fixes https://github.com/flutter/flutter/issues/10547

This patch adds docs for many of the sliver widgets.

This patch adds a notification to SliverChildDelegate that says which
children are alive after each layout. The delegate can use this
information to optimize it's underlying model of the children (e.g., by
discarding models for children that are far outside the live range).

Fixes #9045

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

Covers lib/ in package:flutter.

Previously, the scroll correction code path was untested. This patch
introduces a test for the code path and fixes two issues with it.
There's likely more work we'll need to do in this area to nail down all
the cases.

Fixes #8155

We weren't triggering a relayout when the number of children changed.

Fixes #8001

Also, use CustomScrollView in Shrine and fix a bug with one-line grids
not painting properly due to their reporiting zero paintExtent.

This quantity is actually the layoutOffset of the child, not its scroll offset.

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

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.)

SliverFill fills the remaining space in the viewport with each box
child. We'll use this sliver as a building block for pageable lists.

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.

Add SliverList

A SliverList is a linear layout of box children in a viewport that all
have a common, fixed extent along the scroll axis. The layout is similar
to a SliverBlock but more efficient.