This reverts commit 77ab0b83.

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

What KeepAlive does is different from the Offstage widget. Let's not confuse these two and be more speicific in the wording for KeepAlive.

Fixes #20313.

* sliver separation and test

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Ensure that the _childElements map is properly traversed as a sparse list and not inflated with garbage collected children.

* Add tests to ensure Lists/Tabs with KeepAlive children can make large jumps, don't lose children (including after rebuild).

This should fix https://github.com/flutter/flutter/issues/18731 and I've added a unit test for it.

* Revert "manual rollback of of 18530 (#18531)"

This reverts commit 3f79f8cb.

* Fix the offset calculation

* Add a unit test

We have many hidden children because of iOS accessibility (see also #18410).

For example, in the tile screen of complex_layout app, there could be up to 20 children being painted without this PR. With this PR, only 12 get painted.

This should improve the performance of FL-53 by 40%.

* Fix scrolling of multiple SliverLists and SliverGrids

* Update sliver_fixed_extent_list.dart

* Update sliver_grid.dart

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

* enable lint prefer_foreach

* fix tests

* Make TabBarView/PageView accessible

* review fedback

* Exclude SemanticNodes that are fully covered by an overlapping sliver

* simplification

* formatting

* nits

* add test for center widget

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

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

This patch covers multi-box adaptor and the viewports.

Also, some renames for more accuracy.

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

Fixes #8001

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

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.)