On Windows (manifest-only runfiles), rlocation returns the real file
path in the output tree, not a runfiles tree path. This causes relative
rootUri paths in package_config.json to resolve against the wrong
directory.
Fix by replacing the static package_config.json with a packages
manifest (name/root/representative-file triples). test_runner.dart now
generates the package_config.json at runtime with absolute file:// URIs
derived from rlocation, which works on all platforms.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>