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.

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This PR makes _RenderSlider not be a semantics container. This is so that the FocusableActionDetector in the Slider widget will get to aggregate the semantics information, since otherwise Talkback won't focus the slider because it thinks that the focus node doesn't have anything to say (which it doesn't but the _RenderSlider child does). If the _RenderSlider is a semantics container, then it keeps its speakable information to itself, but it isn't the Focus widget, so when the keyboard focus goes to the focus node, the accessibility focus doesn't move.

Since the _RenderSlider is always wrapped by the Slider widget, there's nothing lost in making it not be a container.

* Adds default cursor and/or mouseCursor property to a number of widgets.
* Adds `MaterialStateMouseCurrsor`.

* Instead of changing test I just flipped flag so the test uses the proper slider

* Use Depracated annotation

* Updated depracation notice

* Updated depracation notice again

* Ignore analyze issues

* flutter analyze .

* ignore analyzer

* // ignore: deprecated_member_use

* Ignoring notice

* indentation

* [Slider] Rebase.

* disabled thumb color with alphaBlend of colorScheme surface

* test remove alphablend

* Change rectangular track to how it used to be

* Chaned to adaptive slider

* blank lines
Co-authored-by: Will Larche <larche@google.com>

This reverts commit e71cf1cd.

This PR adds the linux and windows target platform enum values, along with automatically setting the defaultTargetPlatform to the appropriate value on those platforms.

Fixes #31366

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

* 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

This PR adds TargetPlatform.macOS to the TargetPlatform enum. This allows us to begin implementation of some adaptive UI based on which target platform is desired.

I haven't updated the tests here, that will come in a follow-up PR.

This reverts commit 421bf647.

Was causing severe breakages in Google.

Update the Slider and RangeSlider to the latest Material spec. This introduces an updated track, updated thumbs, updated tick marks, and a new value indicator shape. The old paddle value indicator shape is also updated so that text scaling works consistently.

* Implement system fonts system channel listener