* migrate material to nullsafety

* address review comments

* address review comments

* fix build

Many of the host only devicelab tests were relying on doctor to download android dependencies.

[flutter_tools] remove globals from FlutterValidator, add documentation and move tests to new file (#67234)

Remove globals from the flutter validator class, and refactor the tests into a separate file. Applies some other cleanup like adding doc comments, and making the doctor validator work like it is documented to work - removing the gen_snapshot check if the artifact is not downloaded instead of downloading all android artifacts.

#47161

Add some placeholder docs to the Cocoapods validator and Cocoapods class. Add the injected validator to globals. #47161

move resampler to handlePointerEvent and fix complex_layout_android__scroll_smoothness with PointerEvent (#66745)

[flutter_tools] handle case where file is deleted by other program or running on read only volume (#66708)

* [flutter_tools] handle case where file is deleted by other program

* Add test cases

* Update file_system.dart

* Update file_system_test.dart

* fix import

* make a static on ErrorHandligFS

* add support for no exit on failure

* address comments

* update doc comment to file or directory

Make the logic for locating a local engine path part of its own class, add documentation, and update tests to cover engine source path locating too.

#47161

Fixed a problem with invalid dates when switching back to calendar mode in the date range picker. (#67159)

Improving IV table docs due to confusion in an issue.

Cleans up some undocumented classes and re-organizes the AndroidDevices class to avoid the need for the static testing only member. Adds a script for tracking globals.

Removes usage of global variables, expands documentation, and fixes some formatting inconsistencies. Increased test coverage for android plugin to prove it can determine the embedding version.

[flutter_tools] disable source maps by default for release builds, enable for run and with command line arg (#67331)

Disables source map production by default for build web. For web builds performed as a part of flutter run --release, enable the source maps, or allow force enabling with --source-maps command line flag.

fixes #67328

* Reland dispose images when done (#67100)

Changes since last time:

- Test for CanvasKit image rendering
  (https://github.com/flutter/flutter/pull/67176)
- Fix CanvasKit dispose impl
  (https://github.com/flutter/engine/pull/21555)
- Update internal google3 customer with a problematic ImageStream
  Listener impl (cl/335091311, cl/335459002)

This reverts commit 473358d9.

This functionality ultimately became the single widget reload optimization, which did not require a separate service protocol.

[flutter_tools] support all engine debugging options with FLUTTER_ENGINE_SWITCH environment variables (#67150)

Allow providing all debugging options to the desktop engine via the FLUTTER_ENGINE_SWITCH_ environment variables.

Fixes #66532
Fixes #46005
Fixes #58882

The underling engine changes have already landed for Windows, macOS, but linux is still in progress

Any File-derived IOSink may throw un-handleable async exceptions into the zone, see dart-lang/sdk#43663 . Instead, just write to a file with an append mode.

* Migrate