Updated references to the old button classes to the new ones in comments and documentation for classes in the widgets, services and rendering libraries. (#62099)

https://github.com/flutter/flutter/issues/31931

* state restoration

* added example

* typos and analyzer

* whitespace

* more typos

* remove unnecessary import

* whitespace

* fix sample code

* tests for restorationmanager and restorationid

* ++

* typo

* tests for bucket, part1

* rename tests

* more tests

* finished tests for service layer

* remove wrong todo

* ++

* review comments

* tests for Unmanaged and regular scope

* RootRestorationScope tests

* typo

* whitespace

* testing framework

* tests for properties

* last set of tests

* analyzer

* typo

* dan review

* whitespace

* ++

* refactor finalizers

* ++

* ++

* dispose guard

* ++

* ++

* dan review

* add manager assert

* ++

* analyzer

* greg review

* fix typo

* Ian & John review

* ian review

* RestorationID -> String

* revert comment

* Make primitives non-nullable in prep for NNBD

* 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

This reverts commit c816fabd.

This changes FocusNode.descendantsAreFocusable so that it doesn't allow the enclosing scope to re-focus a node that is a descendant of the node with descendantsAreFocusable set to false.

Because of the order in which the internal state for descendantsAreFocusable was being set, setting it to false was causing a sibling node to be focused when descendantsAreFocusable of the parent was set to false, even though it shouldn't have been focusable, because the enclosing scope would search for a candidate to be focused before the internal state was set to false.

Instead of looping over the children and telling them all to unfocus (and select the previously focused node), this unfocuses the node that has descendantsAreFocusable set to false, with the disposition UnfocusDisposition.previouslyFocusedChild, so that its enclosing scope will look for a previously focused child that isn't part of the subtree being excluded.

This affects how the ExcludeFocus widget behaves when turning on exclude.

The boolean value as to whether the action was found and enabled
is less useful than just returning the result of invoking the action
itself.

`showDialog()` and `showGeneralDialog()` were allowing transparent
colors but then triggering an assertion down the stack while building
the modal barrier. The assertion existed to keep from animating from
transparent to transparent, but there's no need for the assertion,
since we can just treat the transparent case as the same as the null
case -- no animation necessary.

https://github.com/flutter/flutter/issues/61496

The function passes most of its arguments through to
`_DialogRoute`, but it wasn't providing the default
values that `_DialogRoute` was, causing it to override
the defaults will null values.

For #61407 , we need to be able to find all widgets that of a given type. Previously I experimented with using the type name, but of course this does not handles subtypes. The actual check needs to be an is check.

Since there is no way to convert a String to a Type at runtime for use in this check, we can instead evaluate an expression which assigns a closure to a field. The idea for this was inspired by how the dart devtools adds debug functionality to older versions of flutter.

Since the reload feature is not complete yet, adds an integration test which simulates how it will eventually behave