* [flutter_test] handle breaking change to test main . Fixes #67382

Reland: #67155

Fixes analysis error caused by landing of material migration, and g3 error caused by moving of fuchsia lib.

This reverts commit e826442e.

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.