* migrate some material files to nullsafty

* address review comments

* Divider with subheader example update

* fixed issues

* Addressed comments

* fixed issues

Simple docs fix for characters.

* Remove opacity from check and add test

* [Docs] [Icons] Updating dart doc styles to recognize more styles of icons. All the 4 icon styles we support have to be pulled from 4 individual web fonts when we show the html page for api docs.

* Better documentation and that special awful case.

* Couple more places to remove.

Complete the migration to the TextSelectionTheme by deprecating the opt-in flag and text selection properties in ThemeData. (#66482)

* Fixes typos in showDialog documentation

Based on feedback from various desktop developers, and to be consistent between the defaults and the sample code, this PR switches the default for visual density in desktop themes to be compact instead of standard.

It also removes the same setting from the sample code generated by "flutter create" because it is no longer needed now that it is the default.

Wrapping the spacing SizedBox in Flexible.

Fixes a bug when TextField is wrapped in IntrinsicHeight and visual density is below standard.

* wrap LicensePage's loading widget with cardColor

* Set AnimatedSwitcer's duration in LicensePage to zero

* Remove AnimatedSwitcher

* Add test code for checking color is same

Turn the opt-in default for ThemeData.useTextSelectionTheme to true so that everything uses the new TextSelectionTheme by default.

This reverts commit cb92ffc7.

This reverts commit 10a66b19.

This reverts commit fd0554ba.

Fix a bug in the layout of some textfields with constrained height and visual density.

[Material] Fix a jumping animation in the beginning of the extended Navigation Rail transition (#65659)