Ensures all of the libraries that the application side code import are migrated to null safety. full null safety migration is blocked by json rpc 2 and a breaking change to vm service client.

When the developer doesn't have permission to create symlinks, we
display specific instructions, but they were only correct for recent
versions of Windows 10. This improves them by:
- Giving the correct instructions for older versions.
- For recent versions, adds a command that will deep-link into the
  settings application so that developers don't have to figure out
  where/how to enable developer mode.

Fixes https://github.com/flutter/flutter/issues/66973

Use the new single `src/platform.dart` import to pick up what is needed
to implement a test platform from both `test_api` and `test_core`.

* 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