I noticed that AnimatedModalBarrier didn't have the onDismiss callback that ModalBarrier does, and so I added that, and while I was at it, I updated the unit tests to perform all of the tests that are done on ModalBarrier also on AnimatedModalBarrier. The tests are unchanged, other than using AnimatedModalBarrier instead.

This reverts commit 58ad6e1b.

This reverts commit 5fd259be.

This reverts commit d056500b.

This reverts commit 31de052e.

This reverts commit e2490f29.

This reverts commit 35ad43f2.

This reverts commit c26ed03c.

* Migrate to ChannelBuffers.push

* Move TestDefaultBinaryMessenger to its own file

* Apply deprecations and fix usages

* Add dismiss action to modal barrier

* fix test

* fix another test

* NNBD widget test migration

* Add SystemSoundType.alert (supported in the engine in flutter/engine#19970)
* Play system alert sound when user tries to dismiss a non-dismissable modal
  barrier

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

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

[a11y] Make sure RenderFractionalTranslation updates its semantics after the translation field is set (#48985)

* 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