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

* 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 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.

* Added property for expanded padding of expansion panel for solving #24071

* #24071: Renamed default const name for being more descriptive

* #24071: Improved comment of property expandedPadding

* #24071: Improved test by expanding the expansion panel by header tap + precise assertion

* #24071: Renamed property to be more descriptive

* #24071: Adjusted property comment and added missing trailing commas

* #24071: Improved test by checking real exact header size

* #24071: Added test for default header padding

* #24071: Improved comment of expandedHeaderPadding + code style adjustments

* #24071: Added missing trailing comma

* 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

* add missing trailing commas

* add more missing trailing commas

* add more and more missing trailing commas

(Breaking Change) Move some hardcoded pixel values to reusable constants.

- Move `setState` to only be invoked when guarded by `widget._allowMultiplePanelsOpen`, fixing the case for `ExpansionPanelList`

- Remove setting `_currentOpenPanel` to `widget.initialOpenPanelValue` in `didUpdateWidget`, since this should only occur on `initState` and not every time the widget is updated. This fixes the problem for `ExpansionPanelList.radio`

- Added a `didUpdateWidget` condition for when `ExpansionPanelList` changes into `ExpansionPanelList.radio` to open the panel at `widget.initialOpenPanelValue`

- Added test cases for regression, expansionCallback cases, and `didUpdateWidget` transitions between `ExpansionPanelList` and `ExpansionPanelList.radio`

* some space formattings

* always use blocks in if-else if a block is used

* format spaces in for and while

* allow multiline if conditions

* fix missing space

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests