Added ExpansionTileController

* Run dart command to update widget defaults

* Revert the tokens and the template

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Co-authored-by: Qun Cheng <quncheng@google.com>

This reverts commit fd65fd1b.

Fix `ExpansionTile` double tap to collapse/expanded and expanded/collapsed states semantics announcements

* lerp documentation

* Remove Note, Note That from repo

* Improve BorderSide documentation.

* apply review comments

This reverts commit e8eac0d0.

* dart fix --apply

* manual fixes

* Use `curly_braces_in_flow_control_structures` for `material`

* include test/material

* add back removed comments

Move `ListTileTheme` and its tests into separate classes and  add `visualDensity` to the `ListTileTheme` (#100622)

This reverts commit 94ff5202.

This does a cleanup of the examples, removing all of the "section" markers and extra comments that we don't need anymore now that the samples are no longer in the source code. It also removes the --template arguments from the {@tool dartpad} and {@tool sample} directives, since those are no longer used. It converts two examples that I discovered were still embedded into linked examples in the examples folder.

I didn't delete the templates from the snippets config folder yet, because there are still embedded samples in the dart:ui package from the engine that use them. Once dart:ui no longer uses the templates, they can be removed.

I bumped the version of the snippets package to pick up a change that allows removal of the --template argument.

This extracts the sample code out from the API doc comments, and places them in separate files on disk, allowing running of the examples locally, testing them, and building of slightly larger examples.

* 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