[native_assets_builder] Fix pub warning in tests

[dcharkes]

Closes: #149

[auto-submit]

auto label is removed for dart-lang/native/155, due to - The status or check suite build (windows, stable, native_toolchain_c, true) has failed. Please fix the issues identified (or deflake) before re-applying this label.

• The status or check suite build (windows, stable, native_assets_builder, true) has failed. Please fix the issues identified (or deflake) before re-applying this label.
• The status or check suite build (macos, stable, native_toolchain_c, true) has failed. Please fix the issues identified (or deflake) before re-applying this label.
• The status or check suite build (macos, stable, native_assets_builder, true) has failed. Please fix the issues identified (or deflake) before re-applying this label.
• The status or check suite build (ubuntu, stable, native_assets_builder, true) has failed. Please fix the issues identified (or deflake) before re-applying this label.

[coveralls]

coverage: 98.177%. remained the same when pulling 0cb3621 on supress-analytics into fd21f5b on main.

[dcharkes]

Thanks @sigurdm !