* Remove deprecated kind in GestureRecognizer et al

* Fix null case

* DragTarget part 1.

[WIP] Change GestureRecognizer. Sorry.

[WIP] Move from GestureRecognizer to MultiDragGestureRecognizer.

Make it a `Set<int>?`

Get bitwise operations working.

Fix test. Rename to allowedInputPointers.

Convert into a builder.

Improve code with default funciton.

Refactor everything again.

Rename to buttonEventFilter.

Use static function.

Fix analyzer.

Fix private reference.

Use // in private method.

* Fix Renzo request.

* Add `allowedButtonsFilter` everywhere.

* Refactor monoDrag for multi pointer support.

* Fix tests?

* Change default to always true.

* Fix PR comments.

* Completely refactor long press.

* Add forgotten class.

* Revert "Completely refactor long press."

This reverts commit 5038e8603e250e8c928b0f1754fb794b7b75738b.

* Add default value to LongPress

* Refactor doubleTap.

* Relax double tap.

* Write comment in LongPress.

* Use template.

Use `curly_braces_in_flow_control_structures` for `foundation`, `gestures`, `painting`, `physics` (#104610)

* Use `curly_braces_in_flow_control_structures` for `foundation`

* Use `curly_braces_in_flow_control_structures` for `gestures`

* Use `curly_braces_in_flow_control_structures` for `painting`

* Use `curly_braces_in_flow_control_structures` for `physics`

* fix comments

* remove trailing space

* fix TODO style

This is a re-application of https://github.com/flutter/flutter/pull/81884,
but with https://github.com/flutter/flutter/pull/82834 landed first.

Fixes https://github.com/flutter/flutter/issues/81883

They should have been taking a PointerDownEvent.

This change was originally included in #81884, but that
had to be reverted because it broke Google testss. This
sub-part of that larger change needs to be made first in
order to allow for a phased rollout in a way that won't
break Google tests.

https://github.com/flutter/flutter/issues/81883

This reverts commit e88a387b.

* Gesture recognizer cleanup

1) Make OneSequenceGestureRecognizer.addAllowedPointer()
   call startTrackingPointer(), and change subclasses to
   call super.addAllowedPointer() in place of manually
   calling startTrackingPointer().
2) Fix addAllowedPointer overrides to take PointerDownEvent
   where some were taking PointerEvent.
3) Add API documentation to OneSequenceGestureRecognizer
4) Make the following fields in OneSequenceGestureRecognizer
   private with public getters instead of publicly writable:
   `state`, `primaryPointer`, and `initialPosition`.
5) Clean up gesture recognizer state in
   OneSequenceGestureRecognizer.didStopTrackingLastPointer.

Fixes #81883

Currently the framework uses fairly large "hit slop" values to disambiguate taps from drags/pans. This makes sense on touch devices where the interaction is not very precise, on mice however it can feel as if the UI is lagging. This is immediately noticeable on our infra dashboard, where it takes almost half of a grid square of drag before the actual drag kicks in.

One potential solution is to always use smaller constants depending on whether the interaction is mouse or touch based. The only reasonable choice is to use the pointer device kind and not target platform - same platform can have different input sources. This requires exposing the pointer device kind in a few new places in several of the gesture detectors, and using the enum to compute the correct hit slop from an expanded set of constants.

This almost works, however there are a few places (notably ListViews) which uses the touch hit slop as a default value in scroll physics. It does not seem like it will be easy to disambiguate a user provided scroll physics constant from the default and/or adjust it somehow - this might require significant changes to scroll physics which I have left out of this PR.

This PR does not adjust:

kTouchSlop used in scroll_physics.dart's minFlingDistance
kTouchSlop used in PrimaryPointerGestureRecognizer/LongPressGestureRecognizer

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* 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

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

* address review comments

* Fix for nan pressure values and values outside min and max pressure ranges

* Fixes Android keyboard bug

* adds Force Press gesture detector and recognizer