[ffigen] Use package:file instead of dart:io directly

[dcharkes]

Currently FFIgen uses dart:io directly.

This forces our FFIgenPad prototype to use IOOverrides.

If we were to use package:file, we could potentially wire up all File reads and writes using a FileSystem that targets the emscripten file system when we use WASM.

Since this is a more general problem than just our use case, I've gone ahead and file an issue on the Dart SDK to see if others think that would be a good direction to go in:

• [native/js/wasm] Platform independent File I/O sdk#56404

So let's wait on some feedback there before we head in this direction. (marking issue low prio)

Note, it would be tempting to run the the whole of FFIgen with a MemoryFileSystem and make our test suite blazing fast. However, that doesn't work due to libclang wanting to read from the LocalFileSystem.

FYI @TheComputerM @mannprerak2