Adds MediaQuery.maybeOf to replace calling MediaQuery.of(context, nullOk: true), and removes the nullOk parameter. Also changes MediaQuery.of to return a non-nullable value, and removes many instances of the ! operator, reducing the possible places where a null dereference could occur.

* NNBD widget test migration

These widgets are missing from
https://github.com/flutter/flutter/pull/59364

With this change, developers can use clipBehavior for
https://github.com/flutter/flutter/issues/59424

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

fix KeySet.hashCode; enable multiple web tests

* add key to constructors of public widgets

* fix a test

* fix a test

* 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

* 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

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

* 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).

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.

* spelling: accommodate

* spelling: allotted

* spelling: anonymous

* spelling: artificial

* spelling: associated

* spelling: asset

* spelling: button

* spelling: canvas

* spelling: compatibility

* spelling: coverage

* spelling: condition

* spelling: decoration

* spelling: deferring

* spelling: diameter

* spelling: direction

* spelling: displacement

* spelling: dropdown

* spelling: needing

* spelling: environment

* spelling: exited

* spelling: expansion

* spelling: explore

* spelling: families

* spelling: horizontal

* spelling: increment

* spelling: indices

* spelling: internationalization

* spelling: labrador

* spelling: localizations

* spelling: midflight

* spelling: milliseconds

* spelling: minimum

* spelling: multiple

* spelling: multiplication

* spelling: navigator

* spelling: overridden

* spelling: package

* spelling: performance

* spelling: platform

* spelling: porsche

* spelling: position

* spelling: preceded

* spelling: precede

* spelling: precedence

* spelling: print

* spelling: property

* spelling: readily

* spelling: reproducibility

* spelling: rounded

* spelling: scroll

* spelling: separate

* spelling: separator

* spelling: services

* spelling: specific

* spelling: specify

* spelling: synchronously

* spelling: through

* spelling: timeout

* spelling: triangle

* spelling: trivial

* spelling: unusual

* spelling: then

* spelling: vertically

* spelling: visible

* spelling: visited

* spelling: voice

Let translucent Cupertino bars have its scaffold children automatically pad their heights - second try (#13440)

* Let translucent Cupertino bars have its scaffold children automatically pad their heights (#13194)

* Let lists automatically add sliver padding from media query. Translucent nav and tab bars leave behind media query paddings in scaffolds.

* tests

* const lint

* Rename base abstract class to generalized ObstructingPreferredSizeWidget

* review

* More docs and comments from #13317

Revert "Let translucent Cupertino bars have its scaffold children automatically pad their heights (#13194)" (#13317)

This reverts commit 5c4ffa13.

* Let lists automatically add sliver padding from media query. Translucent nav and tab bars leave behind media query paddings in scaffolds.

* tests

* const lint

* Rename base abstract class to generalized ObstructingPreferredSizeWidget

Fixes #11855

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

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