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

This reverts commit a46139a2.

Very simple right-click context menu for Windows and Linux in a Material-esque style.

A very minimal right-click menu for Mac desktop.

This reverts commit 4901744e, which was a minor breaking change that I missed.  Will try to open the PR again following the breaking change process.

Adds MediaQuery.maybeOf to replace calling MediaQuery.of(context, nullOk: true), and removes the nullOk parameter. Also changes MediaQuery.of to return a non-nullable value, and removes many instances of the ! operator, reducing the possible places where a null dereference could occur.

Enables custom text selection menus by allowing selectionControls to be passed to TextField et. al.

* enable null_check_on_nullable_type_parameter and tighten_type_of_initializing_formals

* fix CI

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

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

Create a new TextSelectionTheme for text selection properties.

* Fix even more unresolved doc references in material

* ++

* ++

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This PR adds the linux and windows target platform enum values, along with automatically setting the defaultTargetPlatform to the appropriate value on those platforms.

Fixes #31366

Use strut box heights to calculate selection rectangles in order to ensure that they remain within visible bounds (#50354)

* Feature: allow minLines in SelectableText