[native_assets_builder] Use `ProcessManager` and `FileSystem`

[dcharkes]

flutter_tools unit tests are split into hermetic and permeable tests.

The hermetic tests mock the file system and process invocations.

• Mocking processes: https://pub.dev/packages/process
• Mocking file system: https://pub.dev/packages/file

The native_assets_builder does not use these abstractions, and will try to read/write the local file system and do process invocations.

This means we cannot write hermetic unit tests for the native assets feature, and that we have to always skip the check for native assets in the hermetic unit tests.

[dcharkes]

All our dependencies need to also support these two packages.

• package:package_config does not, but it doesn't use the file system if a loader argument is provided.
• The other dependencies don't use the file system or process invocations.

Moreover unit tests would need to mock the process output and file system changes. Some interesting cases:

• dart pub get creating a package config file.
• dart pub deps --json printing a JSON graph to stdout.
• build.dart invocations.
  • Generating various build_output.yaml files, both valid and invalid ones.
  • And these files need to contain the right time stamps to make the caching logic work.
  • Generating various dynamic and static library files.
  • Simulating build failures by non-zero exit codes.

Simulating all these things in tests might not be the best thing to do. So it's not clear that this will actually improve the code.

For the context of flutter_tools it might be best to mock this package instead of all the file system operations and process calls in tests.