[coverage] Support native assets in `test_with_coverage`

[liamappelbe]

Tests run under test_with_coverage can't seem to locate their native assets. This will become more of an issue when Dart 3.10 hits stable and people start using native assets in their packages. I'm already hitting it here: dart-lang/native#2329

[liamappelbe]

The issue is much more specific than I originally thought. I'm not able to reproduce it in a toy example. The code_assets examples I tried work fine under test_with_coverage, including from a cold start where they have to build the asset first.

So it must be something specific about my package:objective_c migration that's broken.

[liamappelbe]

New hypothesis: if a library you're importing uses native assets, those will fail to load when running via test_with_coverage. I'll test this tomorrow.

[liamappelbe]

Nope, it's not just a matter of depending on a package with native assets. Toy examples using that pattern also don't repro the bug.