Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

Fixes: "FloatingActionButton.extended's isExtended property if false should show icon, not label" (#72431)

* Fix extended FAB with isExtended set to false showing both icon and label

* Update floating_action_button.dart

* Fix default hover elevation

* revert letter spacing change

* add test

* Update floating_action_button.dart

* Update floating_action_button_test.dart

My yearly pass to fix spelling errors in the API dartdocs.

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

* Fix even more unresolved doc references in material

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* 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`.

* Uses a special cursor value MouseCursor.defer to mark deferring. MouseTrackerAnnotation.cursor is now non-null. The mouseCursor argument of widgets or render objects can be null, which indicates using the default value.
* Moves SystemMouseCursors.uncontrolled to MouseCursor.uncontrolled.
* Changes how MouseCursor.debugDescription is defined. Previously MouseCursor.toString returns $runtimeType($debugDescription), while now it returns $debugDescription. Implementations of classes are updated, except for the ones of MouseCursor.defer and MouseCursor.uncontrolled are simply "defer" and "uncontrolled".

Adds the basic framework for the mouse cursor system. 

* Adds MouseRegion.cursor
* Adds SystemMouseCursors
* Adds mouseCursor to some widgets

This reverts commit 2132a0c7.

This reverts commit 69080e07.

This PR modifies the existing API docs samples to use DartPad so that all of the samples are now interactive apps on the API docs site.

It also removes the restriction for the max width of the description area so that the dartpad region can expand horizontally.

I updated the first paragraph on the API docs to indicate that Flutter is more than just mobile now (same text as the README.md at the top level).

I modified a few of the examples so that they looked nicer, and fit better on the page.

I added the sample description text above each DartPad instance, since that often defines the context of the example.

I removed animations and images when they were redundant with the sample content. There were a few that made sense to keep, so I did.

Step 2 of 2: Remove Flutter's FloatingActionButton dependency on ThemeData accent properties (#46923)

Step 1 of 2: Warn about Flutter's FloatingActionButton dependency on ThemeData accent properties (#48435)

* 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

I noticed that we were pretty inconsistent with the way that we checked the value of clipBehavior in the framework, so I normalized the usages and updated docs where necessary.

This is a breaking change if you used to pass null explicitly to FlatButton, OutlineButton or RaisedButton constructors, expecting to get Clip.none. It will now assert if you do that. Existing implementations that pass null implicitly by not specifying clipBehavior won't need to change their call sites. It always implicitly defaulted to Clip.none before, and it will continue to do that, it's only places where it was explicitly set to null in order to get the implicit default that it will fail.

Add an autofocus parameter to widgets which use Focus widget internally, and update related docs.

This will allow developers to request that a particular widget be automatically focused when shown.

* [Material] Add splashColor param to FAB and FAB ThemeData