Decouple more tests from dart:io

[brianquinlan]

There should be no semantic changes to the tests. Instead of testing package:io_file against dart:io, the updated tests run each test case twice: once using dart:io for setup and verification and once using package:io_file for setup and verification.

Not all tests can be expressed using package:io_file (yet).

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[kevmoo]

Have you looked at https://pub.dev/packages/test_descriptor ?

[brianquinlan]

That's neat! I hadn't seen that package before.

I don't think that I can use it though because it depends on dart:io. The purpose of this TestUtils refactor is to (eventually) divorce the tests from dart:io completely so that you can verify fakes (where the file system objects will exist only in memory).