This reverts commit 2132a0c7.

This reverts commit 69080e07.

* implicit-casts:false in flutter/lib/src/rendering

* address review comments

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* 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.

Fixes #11855

Applies comments I missed from https://github.com/flutter/flutter/pull/10904.

Also includes minor doc fixes.

* make @immutable const

* fix build

There are more places we can use this annotation, but this patch just gets us
started.

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

Previously they tried to compute an integer target end index, but
integers can't represent infinity. Now we use null to represent
infinity.

Also, fix some similar issues with grids.

Fixes #8398

Most of these docs were already in place, but this patch adds the remaining
missing ones.

Also, some renames for more accuracy.

Covers lib/ in package:flutter.

Fixes #7734

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

Fixes #8001

This patch improves PageView to the point where we can use it in the date
picker. Specifically, you now get onPageChanged notifications and you can
control which page is visible using a PageController.

This patch converts the Shrine home page to using a sliver-based grid.
This required using a CustomScrollView to mix the block at the top with
the grid below.

We now create a GridLayout object that memoizes the layout information
for a given SliverConstraints. This approach is both more efficient
(because the delegate no longer needs to re-solve its layout for each
child) and allows for more code sharing between delegates.

The SliverGridRegularTileLayout will also be useful for the date
picker.

Also, document the grid delegates.

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 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.)